122 



tissii called the SPONGY PARENCHYMA. In contrast to the palisade 

 cells, the cells of the spongy parenchyma are less abundantly 

 supplied with chlorophyll ; they are also much more irregular in 

 shape, and have large intercellular air-spaces between them. The 

 palisade cells are elongated in the 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 paren- 

 chyma, 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 stomata of the lower epidermis. HABER- 

 LANDT ( m ) has estimated that to every square millimetre of surface 

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



FIG. 132. Transverse section of a leaf of Vagus sylvaticc. ep, Epidermis of upper surface ; ep", 

 epidermis of under surface ; ep"', elongated epidermal cell above a vascular bundle ; pi, palisade 

 parenchyma ; s, collecting cells ; sp, spongy parenchyma ; k, idioblasts with crystals, in k' 

 with crystal aggregate; st, stoma. (x 300.) 



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. 132). In this way 

 several palisade cells come into direct contact with a single expanded 

 cell of the spongy parenchyma, which thus functions, apparently, as 

 a collecting cell for a group of palisade cells. The products of 

 assimilation are passed on from the collecting cell through the 

 spongy parenchyma, to be finally carried to the mesophyll sheath 

 surrounding the vascular bundles. The sheaths serve as a 

 conducting tissue towards the stem. 



At the base of the lamina the tissues close together and pass into 

 the leaf-stalk, where one is present. The dorsiventral structure 

 becomes less marked in the petiole. The cells are mostly elongated 



