DORSIVENTRAL LEAVES 



in 



The spongy cells also serve to conduct elaborated food-materials 

 in various directions to the adjacent veins, a function to which 

 they are well suited by virtue of their irregular shape. 



The structure of the mesophyll just described is characteristic 

 of dorsiventral leaves generally, the following being the chief 

 modifications. The palisade tissue not uncommonly consists of 

 several layers (e.g. Wallflower, Holly, Fig. 43, A), a feature 

 especially encountered in leaves exposed to strong illumination, 

 whilst leaves developed in dull light may have little or no palisade 

 tissue . In 

 some cases 

 (e.g. the 

 House-leek) 

 the assimila- 

 tory cells have 

 quite a differ- 

 ent form, being 

 elongated 

 parallel to the 

 midrib, whilst 

 in transverse 

 section they 

 appear more or 

 less rounded. 

 Plants grow- 

 ing in dry 

 sunny situa- 

 tions often ex- 

 hibit a second 

 epidermis. 



FIG. 53. Transverse section through part of the leaf 

 of the Elder (Sambucus) showing the upper 

 epidermis (e.) ; the arm-palisade cells (a.p.) ; the 

 collecting cells (c.c.) ; and the spongy parenchyma 

 (s.p.). i., intercellular space. 



layer of colourless cells beneath the upper 

 The cells of this hypoderm (p. 95, Fig. 43, Hy.) are 

 usually thick-walled, and consequently form with the epidermis 

 a strong surface skin, besides further screening the underlying 

 chloroplasts from excessive illumination. 



Variegated leaves usually exhibit a similar structure to that 

 of the normal foliage of the same species, except that localised 

 areas show no development of chlorophyll ; these areas in con- 

 sequence appear white or yellow. It need hardly be said that 

 the structure of reduced foliage-leaves (e.g. scale-leaves of rhi- 

 zomes, bud-scales, etc.) is of a much simpler character. Such are 



