We tested Charlton’s hypothesis (1990) that stomata are present and patterned in linear cell aggregations using the monocot Tradescantia. We examined the following features of the leaf epidermis in support of this theory: linear groups (strings) of stomatal complexes and of epidermal cells were sought in immature and mature regions of entire leaves; the lengths (in cell number) and incidences (numerical occurrence) of both string types were determined; the uniformity and progression of stomatal differentiation within strings were studied; physical characteristics of differentiating strings within cell files were measured. Undifferentiated epidermal cells from the leaf base were stained with DAPI to reveal precursors of stomatal strings immediately proximal to the stomatal initial region. The results indicated that the Tradescantia epidermis in the leaf blade consists of linear groups of stomata and epidermal cells, which did not change in cell number nor incidence during development. The incidence of stomata by length was nonrandom. Although incidence decreased with string length, the decline was not linear nor exponential. Stomatal strings show cell cycle synchrony in DAPI staining of stomatal precursors and synchrony of stomatal differentiation within a string. The irregularity in the length of the stomatal development region, and each differentiation stage in it, by cell file was consistent with the variation in string length and unity in string development. The evidence supports Charlton’s hypothesis that cells are patterned based on their position in the cell cycle and that linear groups of stomata reflect cell lineages, which maintain a degree of cell cycle synchrony.
The concept of subsidiary cells is discussed in the light of the existing literature correlating ontogeny arid mature structure. The protodermal origin of each epidermal component is illustrated and explained and the status of the ontogenetic classifications of these components has been analysed and discussed. Terminology is also defined and discussed.
The leaves of Larrea show numerous structural adaptations that extend their tolerance to drought and desiccation.
The epidermis consists of a single layer of small compact and heavily cutinized cells. Most epidermal cells contain dark staining deposits of waterproofing resins. Long epidermal hairs and stomata are present over the entire leaf surface. Stomata are roofed by epidermal cells with ledge like extension of cutin and overlay large substomatal chambers.
Two or three rows of tightly packed palisade mesophyll are present below the adaxial and abaxial epidermis. Spongy mesophyll is reduced to a narrow central band that supports the vascular bundles. In some preparations resins may be seen lining the stomatal cavities and coating the outsides of palisade cells. Idioblasts containing crystals of calcium oxalate are abundantly distributed through both mesophylls.
Small, centrally located vascular bundles span the breadth of the leaf. The bundles are collateral and closed with xylem of vessels and tracheids towards the adaxial(upper) surface and phloem of sieve tubes and companion cells towards the abaxial (bottom) surface. Cambium is not present.
Each vascular bundle is wrapped by a bundle sheath and supported towards adaxial and abaxial surfaces by small caps of supportive sclerenchyma. Abaxial caps are especially well developed.
Fig. 10.3-10. Transverse section of oleander leaf (Nerium oleander). Oleander leaves are a favorite in plant anatomy laboratories because they demonstrate a placement of stomata that has ecological significance. The arrows indicate three stomatal crypts: the crypts are large chambers in the mesophyll, covered with an epidermis that contains stomata as well as trichomes (hairs) that project into the crypt. The epidermis on the exposed surface of the leaf – between crypts – lacks stomata. The narrow opening between the crypt and the atmosphere, combined with the presence of trichomes, causes the air inside the crypt to be rather immobile, even if there is a strong wind blowing over the leaf. Any water molecule that diffuses out of a stomatal pore will spend so much time that there is a high probability it will diffuse back into one of the stomata in the crypt. See following figures for higher magnifications