Bbtts. — Eosette Plants at Cass. 



261 



cells. The cell-wdlls are thin in both upper and lower surfaces, especially 

 the latter, and here the cells, as seen in surface view, are even more irre- 

 gular in outline than those of the upper (figs. 13 and 14). Stomata, in 

 which the guard-cells are very small, are found on both surfaces, but are 

 more numerous on the lower ; on the upper surface there are about 65 

 per square millimetre, and on the lower about 105. A few of the cells of the 

 upper epidermis (fig. 12) are produced into long, unicellular, thick-waUed 

 hairs, the walls of which are covered with small excrescences of calcium 

 oxalate. On both surfaces is a thin cuticle. 



The chlorenchyma is differentiated, although in parts not too clearly. 

 The palisade parenchyma consists of large, thin-walled, more or less oval 

 cells which vary in size and which contain numerous small oval chloro- 

 plasts. This layer is usually only one cell deep. The spongy tissue, which 

 occupies the greater part of the leaf, consists of oval or irregular cells which 

 are loosely arranged, so that there are large intercellular air-spaces. This 

 tissue also contains numerous chloroplasts. 



The vascular bundles are small, and each is surrounded by a little 

 thin-waUed colourless parenchyma. The xylem consists of vessels which, 

 in section, are small, circular, and thick-walled ; associated" with the xylem 

 s a small amount of xylem parenchyma. The phloem is of the usual form. 



The leaf is thickened at the midrib, where there is an aqueous tissue, 

 consisting of large, roundish or irregular thin-walled cells, in which there 

 are a very few chloroplasts. 



Fig. 15. — Cardamine heterophylla. Transverse section of petiole ( X 72). 

 a, furrow of upper surface ; b, chlorenchyma ; c, aqueous tissue. 



Petiole (fig. 15). — The petiole is convex on the lower surface, and on the 

 upper is more or less flattened, with a groove running down each side of the 

 midrib. The epidermis is the same as in the leaf, consisting of small cells, 

 together with much larger, somewhat projecting cells. Stomata are found 



