DORSIVENTRAL LEAVES 



III 



The spongy cells also serve to conduct elaborated food-materials 

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

 the}' are ^^■ell suited bv virtue of their irregular shape. 



The structure of the mesophvl! just described is characteristic 

 of dorsiveniral leaves generallv, the foUowing being the chief 

 modifications. The palisade tissue not uncommonl\' consists of 

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

 especiall}' encountered in leaves exposed to strong illumination, 

 whUst leaves developed in dull light may have httle or no pahsade 

 tissue. In 

 some cases 

 [e.g. the 

 House -leek) 

 the assimila- 

 torv* 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 dn." 

 sunny situa- 

 tions often ex- 

 hibit a second layer of colourless cells beneath the upper 

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

 usuallv thick-walled, and consequently form with the epidermis 

 a strong surface skin, besides further screening the underMng 

 chloroplasts from excessive illumination. 



Variegated leaves usually exhibit a similar structure to that 

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

 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 fohage-leaves [e.g. scale-leaves of rhi- 

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



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 parench3-ma 

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



