SECT. I 



MORPHOLOGY 



115 



milating foliage leaves, especially of shade-loving plants, may have a 

 similar uniform structure ; but they are usually more complicated, 

 and exhibit a difference in the structure of their upper and lower 

 sides (Fig. 130). They are, accordingly, dorsiventral, and, in corre- 

 lation with this difference in structure, their two surfaces react dif- 

 ferently toward external influences. In such dorsiventral structures 

 the upper epidermis is succeeded by one or more layers of cylindrical 

 parenchymatous elements elongated at right angles to the surface, and 

 known as the palisade cells. These are especially rich in chlorophyll, 

 and contain only small intercellular spaces. Adjoining the palisade 

 parenchyma, and extending to the epidermis (ep") on the under surface 

 of the leaf, is a loose tissue called the spongy parenchyma. In contrast 

 to the palisade cells, the cells of the spongy parenchyma are less 



k , z 



Fig. 130. — Transverse section of a leaf of Fayus syliv.tica. ep, Epidermis of upper (ventral) 

 surface ; ep", epidermis of under (dorsal) surface ; ep"', elongated epidermal cell above a 

 vascular bundle ; pi, palisade parenchyma ; t,, collecting cells ; sp, spongy parenchyma ; /.-, 

 idiohlasts with crystals, in 1:' with crystal aggregate ; si, stoma. ( x 360.) 



abundantly supplied with chlorophyll ; they are also much more 

 irregular in shape, and enclose large intercellular air-spaces. The pali- 

 sade cells are elongated in the same direction in which the rays of 

 light penetrate the leaf-lamina, and by this means are particularly 

 adapted to their special function of assimilation. The spongy 

 parenchyma, on the other hand, is arranged to facilitate the free 

 passage of gases, and to that end develops large intercellular spaces 

 in direct communication with the gtomata of the under epidermis. 

 Haberlandt has estimated that to every square millimetre of surface in 

 a leaf of Bicinus communis there are, in the palisade cells, 403,200 

 chlorophyll granules ; in the cells of the spongy parenchyma only 

 92,000 ; that is, 82 per cent of all the chlorophyll granules belong to the 

 upper surface of the leaf, and only 18 per cent to the under side. 

 The palisade cells are often arranged in groups, in which the lower ends 

 of the cells of each group converge (Fig. 130). In this way several 



