By using the "inner skin" the biology teacher focuses the student to the study of the plant cell, and one plant tissue, the epidermis, without having to divert their attention to explanations about the stomata, its structure and its function, nor on the homology between a bulb scale and an aerial leaf.
It is a useful trick. Normally the other "onion skins" usable for scholarly lectures the outer papery layers or tunics , are reserved for more advanced courses, in which it is studied the secretion by plants of different chemicals, especially calcium oxalate , which form clearly visible and very interesting crystal structures.
To see them, cut with scissors a 1. If colored, the tunic would diffuse some color it's a useful pigment that people use to tinge textiles or Easter eggs and become pliable.
The more pigment lost, the better image you have. It only remains to make a wet mount. Really the tunics are not an epidermis. They are the compressed dry remnants of one, two or more exterior scales of the bulb. Some focusing would convince you that the crystals are embedded in the dry mesophyll.
This is why also you can see, as in any leaf, the veins with the vessels that conduct water through the scale. Fig 11 to 13 shows crystals of oxalate found in the tunic of a white onion. Logitech Fig Some additional pictures of individual crystals in the tunic of a white onion and a shallot.
Clips from 2 Mpx pictures taken with the Canon Powershot A In all the pictures crystals are embedded in the mesophyll, and are seen through the superficial layer of the epidermis.
As my microscope is not a DIC with the power to make net optical sections, the images are not very sharp. Shadows of the limits of the long cells of the epidermis are seen, out of focus.
In other plants are also common calcium carbonate crystals. Why do I know that the onion ones are calcium oxalate? Because it's said by microscopists from the 18 century to now that is not difficult to establish a diagnosis, using two chemical reactions, which can be controlled with the microscope:.
In principle it is interesting that, fortunately for us microscopists, these crystals are insoluble in water, alcohol, acetic acid, or even in the hypochlorite, which is commonly used to destroy the cytoplasm and clean plant sections to be colored and mounted. This is why we can see them easily.
Oxalate crystals are insoluble in acetic acid, but soluble without effervescence in hydrochloric acid. The crystals of carbonate by contrast are soluble in acetic acid and hydrochloric acid, with effervescence. As in my samples I submit to, and they resist the acetic acid, I can say they are oxalate, but What are the calcium oxalate crystals for? And possibly they are also a form to sequester, and to turn inoffensive, the oxalic acid produced by the plant metabolism itself.
The preparation I made to see the crystals of the white onion, give me an unexpected gift. The huge variety of plant shapes is achieved through a combination of oriented cell divisions and anisotropic cell expansion.
Anisotropy in cell growth means that a plant cell expands at different rates in different directions, and it is characterized by the direction and the degree of anisotropy. The former indicates the direction in which the maximal expansion rate occurs while the latter specifies the degree to which this maximal rate exceeds the minimal rate at a given time point Baskin et al. Due to methodological problems the role of stress distribution in the control of growth anisotropy is still poorly investigated, whereas the role of anisotropic cell wall structure is well documented Baskin, Cellulose microfibrils are considered as key elements conferring structural anisotropy and reinforcing the wall parallel to their orientation.
As a result, the cell wall will extend more easily transverse to the net cellulose alignment Taiz, Several lines of evidence confirm that the specific alignment of cellulose microfibrils controls the direction of expansion anisotropy. First, it is deduced from the correlative data indicating that plant cells usually grow faster in the direction transverse to the net cellulose orientation in their walls Sugimoto et al. Secondly, biomechanical experiments with isolated cell walls show that they are more extensible in the direction transverse to the net microfibril alignment Richmond et al.
Finally, suppression of cellulose synthesis genetically or by inhibitors usually disorganizes the microfibril orientation in the cell wall and disturbs normal anisotropic expansion Desprez et al. The role of cellulose microfibrils in the control of the degree of anisotropy is still debatable.
Originally it was postulated that the degree of expansion anisotropy in Nitella cell walls depends on the extent to which microfibrils are aligned Green, The higher the microfibril alignment in the cell wall the more anisotropic growth takes place.
However, this hypothesis was questioned for higher plants since strong changes in the degree of expansion anisotropy were not associated with changes in the extent of microfibril alignment Baskin et al.
There were several explanations for this discrepancy. In line with this assumption genetic evidence indicated that different non-cellulosic factors were responsible for growth perpendicular and parallel to the net cellulose orientation Wiedemeier et al.
According to an alternative point of view the degree of expansion anisotropy is controlled by a fine arrangement of cellulose microfibrils that is difficult to detect using routine methods of analysis. However, this alignment may be different in adjacent regions of the same cell wall or between walls from different cells in a tissue Baskin et al.
Thus it was hypothesized that the extent of global but not local cellulose alignment defines the degree of expansion anisotropy. Finally, microfibril length, which is difficult to measure, was considered as a factor affecting the degree of expansion anisotropy Wasteneys, Of two cell walls having the same cellulose alignment but different microfibril length, the one containing shorter microfibrils will expand less anisotropically due to the lower resistance of short microfibrils to slippage past each other which increases the rate of expansion parallel to the net cellulose microfibril orientation.
This uncertainty concerning the role of cellulose microfibrils in the control of the degree of expansion anisotropy is partly explained by the lack of convenient experimental models. Most studies have been done on systems designed to unravel the mechanisms of axial growth, principally on roots.
As these rapidly growing organs have an extremely high degree of growth anisotropy, accurate measurement of this characteristic is quite difficult, and only a few papers report on successful measurements Liang et al. Thus a strong need exists for a convenient model that allows measurement of the degree of growth anisotropy and for checks to be made of whether cellulose microfibrils or other factors control this. Ideally this model should provide: i reliable quantification of the degree of growth anisotropy; ii easy determination of cellulose microfibril orientation and the extent of their alignment; and iii the ability to measure the wall extension in vitro in different directions to the growth anisotropy and to the cellulose orientation, giving an estimate of the wall extensibility, the key parameter defining the growth rate.
Good correlations between differences in the wall extensibility measured transverse and parallel to the cellulose orientation, the degree of growth anisotropy, and the extent of microfibril alignment would show that the degree of growth anisotropy is controlled by cellulose microfibrils. These data, however, cannot be extrapolated to higher plants due to considerable differences in the wall composition and molecular mechanisms of cell expansion of higher plants and charophycean green algae Cosgrove, ; Popper and Fry, ; Proseus and Boyer, This work demonstrates that the adaxial epidermis of onion Allium cepa bulb scales can be used as a novel model for unravelling the mechanisms of control of anisotropic expansion in higher plants.
By using the correlative approach described above, the model allows checks to be made on whether cellulose microfibrils or other factors define the degree of growth anisotropy. The onions A.
Bonkajuin used for this study were planted in a field at the end of March as sets small bulblets. The developing bulbs were collected at week 6, 8, 10, and 12 after planting and are referred to in the text as stages I, II, III, and IV, respectively. Five bulbs were chosen for analysis from the material collected at each stage in such a way that their diameters were: i approximately equal and ii close to the maximal for that stage.
The first segment of each scale was used for cell size measurements. The second segment of each scale was used for analysis of the net cellulose orientation. The third segment was excised only from the sufficiently large scales the majority of scales at stages III and IV and used for extensiometry. As the adaxial epidermis adheres very weakly to the underlying parenchyma in young onion bulbs, the epidermal strips used here always consisted of one cell layer.
Numbering of the scales reflects the actual order of their formation during bulb development. Number 1 is the most external scale at stage I, being the first green leaf-carrying scale formed; the higher numbers were assigned to subsequent more internal and younger scales.
Despite the fact that scale 1 was dead after stage I, this leaf-carrying scale was clearly visible up to stage IV. Using scale 1 as a reference allows establishment of the unified scale numbering at stages I—IV. All cells have a more or less rectangular shape in the plane of the epidermis. The degree of growth anisotropy was calculated as a ratio of cell growth rate in the two dimensions. The net or mean orientation of cellulose microfibrils in the outer periclinal wall of the adaxial epidermis was determined using polarization confocal microscopy and fluorescent Congo Red staining as described previously Verbelen and Stickens, ; Suslov and Verbelen, Given that the four areas were studied in each scale of a bulb, and a total of five bulbs were analysed, the reported data on the net cellulose orientation are based on 50 repeats 10 repeats in each scale of a bulb.
The dichroic fluorescent dye Congo Red preferentially absorbs light directed parallel to the dipole moment of its chromophoric group. In the case where the cellulose fibrils have a preferential orientation, Congo Red fluorescence will be maximal when the vector of the polarized exciting laser beam is parallel to the net cellulose orientation Fig.
When the vector is perpendicular to the net microfibril orientation, fluorescence intensity will be minimal Fig. In the case of a net random cellulose orientation, wall fluorescence intensity will be equal for all orientations of the polarized light vector Fig. Onion bulb scales and their adaxial epidermis. A—D Polarization confocal micrographs of Congo Red-stained adaxial epidermis cell walls demonstrating the mean cellulose microfibril orientation. The intensity of fluorescence is colour-coded low, blue; high, red.
The polarization vector of the laser beam is indicated by arrows. The intensity of fluorescence is maximal when the vector of polarization is parallel to the net cellulose orientation, and it is minimal when the vector of polarization is perpendicular to the net microfibril orientation.
Accordingly, cell walls from scale 6 at stage I A, B have the net transverse orientation of microfibrils to the bulb's axis which is parallel to the long axes of cells while cell walls from scale 2 at stage I C, D have the net longitudinal cellulose orientation.
E—H Polarization confocal micrographs of Congo Red-stained adaxial epidermis cell walls demonstrating cellulose microfibril reorientation in the direction of strain in vitro. As a result, microfibrils realigned in the direction of strain, and their orientation became random G, H.
I—K General view of onion scales at different stages of development: I the youngest scale soon after its formation, J the same scale at the next early stage of development, and K the oldest live scale in a young onion bulb. Upper and lower margins of the scales are marked with arrows. In order to show the scales in I and J all other scales were removed, the remaining scales were cut longitudinally across their green leaves into two symmetrical halves, and their concave sides with the adaxial epidermis facing the viewer were photographed.
N Hand-made sections embedded into a LX resin and stained with toluidine blue [0. The extent to which cellulose microfibrils were aligned with respect to the cell's long axis, which is parallel to the axis of the bulb, is expressed as the axiality ratio AR. This is the ratio between the fluorescence intensities quantified using Adobe Photoshop 4. The AR equals unity if the net cellulose microfibril orientation is random Fig. It will have values above Fig.
The in vitro extensibility of onion epidermal cell walls was studied with a custom-built constant-load extensiometer as described previously Suslov and Verbelen, The AR in the outer wall of a randomly chosen cell in the central part of a transverse epidermal strip prepared as for extensiometry was determined as described above. The epidermal tissue next to the chosen cell was marked with black ink to allow easy tracing of the cell taken. Subsequently the strip was removed from the extensiometer, and the AR of the chosen cell in relaxed state was determined again and compared with its initial value.
Several external scales of the bulblets planted in the field do not produce green leaves. An easily separable envelope of these dead and partially disintegrated scales marks the border of a new developing bulb. It is composed of leaf-carrying scales whose adaxial epidermis was studied in the present work. The total number of leaf-carrying scales Fig.
Formation of new scales stops between stages III and IV as the total number of live scales at stage IV from seven to nine is equal to that found in mature onion bulbs of the cultivar studied D Suslov et al. The oldest external leaf-carrying scales dying at stages II—IV become dry and form a multilayer envelope protecting the underlying live scales.
The range indicated reflects the fact that bulbs have different numbers of scales at stage IV. At stage I each of the leaf-carrying scales represents a tube with the same diameter along its length Fig. A localized growth in girth close to the base of these tube-shaped juvenile scales starts at stage II and highlights the beginning of formation of a bulb.
The subsequent progressive outgrowth results first in the formation of an axially extended bulb stage III and, finally, leads to a spherical bulb stage IV Table 1. The early onion bulb development studied here involves all stages from the first signs of bulb formation to a small spherical bulb containing the final number of live scales typical for this onion cultivar. Cell dimensions in any plant organ are defined by the balance of their expansion and division.
The former increases cell size while the latter decreases it Green, In the absence of cell divisions the rate and the direction of growth in a plant organ are proportional to the increase in cell dimensions. Staining the adaxial epidermis of onion bulb scales with acridine orange did not reveal mitotic nuclei in any of the scales studied results not shown.
Therefore, the data on cell dimensions in the epidermis at the successive stages of development can be used to calculate the average rate of expansion in length and in diameter, and to determine the direction and the degree of growth anisotropy.
On cross-sections the adaxial epidermis cells look more or less like circles Fig. In longitudinal sections they have a rectangular to slightly irregular elliptical shape not shown. Although adjacent cells in the same epidermis can differ considerably in size Fig.
A plot of mean cell dimensions Fig. At each stage I—IV there is a gradual increase in cell size from the youngest to the oldest scales. Between stage I and II cell growth is small or nil. From stage II on the increase in both cell dimensions was much higher. Cell dimensions and growth rate in the adaxial epidermis of scales during onion bulb development. Profiles of cell length and growth in length A , and cell diameter and growth in width B.
Numbers I—IV denote successive stages of onion bulb development. Only halves of error bars are indicated for some data points for clarity of presentation. A detailed analysis of these data demonstrates that cell expansion in the adaxial epidermis is mostly anisotropic Fig. Moreover not only the degree of growth anisotropy but also its direction changes during onion scale development Table 2.
The youngest scales grow predominantly in length Fig. The fact that the intermediate scales 4 and 5 do not grow at the interval I—II Fig. Analysis of many onion bulbs at stages I—III reveals very few young scales having the intermediate length Fig. Apparently the initial elongation of scales is relatively rapid and transient, which makes it more difficult to be measured reliably within the time scale used in the present study.
This rapid axial extension of scales is followed by a slow expansion predominantly in width, leading to onion bulb formation. Cellulose microfibril orientation and direction and degree of growth anisotropy in the adaxial epidermis of onion bulb scales.
Cellulose orientations are shown relative to the long axis of a bulb. The degree of growth anisotropy is calculated on the basis of growth rates given in Fig. The direction of growth anisotropy is shown in bold when it agrees with the net microfibril orientation. Two pairs of data obviously inconsistent with the idea that cellulose microfibril alignment controls the degree of growth anisotropy are underlined and double underlined.
Cellulose microfibril orientation was studied in the outer periclinal walls of the epidermis cells using polarization confocal microscopy and fluorescent Congo Red staining.
In Fig. The outer periclinal wall is considerably thicker than the other cell walls in the tissue. When stained with the cellulose-specific Congo Red, it looks like a bright band bordered by two fainter bands Fig. The outer cell walls stained with propidium iodide or with toluidine blue binding to various wall polymers look like homogenous bands of the same thickness Fig. This shows that Congo Red penetrates well through the whole thickness of the outer epidermal wall and therefore gives information on the mean cellulose orientation across all wall layers.
The faint bordering bands are probably caused by a limited Congo Red diffusion from the wall into the surrounding solution. The profiles of mean cellulose orientation in the scales at different stages during bulb development are shown in Fig. This orientation is expressed as the AR and demonstrates striking changes during growth. Other scales repeated at least part of this scenario during their development Fig.
That depends on how you define layers. You have two main layers to the integument or skin, the epidermis and the dermis. The epidermis itself is divided into four or five layers or strata , depending on where in the body it is found. There are two layers of skin, the epidermis and the dermis. The epidermis has five layers; straum germinativum, stratum spinosum, stratum granulosum, straum lucidum, and stratum corneum. The skin has two distinct layers.
The outer layer is the epidermis. The epidermis is comprised of five layers. The inner layer is the dermis. The dermis is anchored to a subcutaneous layer, but it is not considered part of the skin. It depends on the size of the onion. The skin is actually made of two layers, the epidermis and the dermis.
The outer epidermis has five layers; the stratum germinativum, stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum. One cell thick. Onion, cabbage, lettuce. Skin usually has three main layers - the exterior epidermis; the middle dermis; and the interior hypodermis. Log in. Study now. See Answer.
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