PISUM SATIVUM 343 



The outermost layer of palisade cells, sometimes referred to 

 as the Malpighian layer, consists of elongated cells which may 

 be from 60 to 100 /x long, and are further characterized by the 

 presence of a narrow light line immediately below the thin cuticle. 

 The cells are thick-walled and the lumen is much broader at the 

 inner end of each cell than at the outer where it is very narrow. 

 The second layer consists of the column cells or osteosclerids, 

 which have a characteristic I-shaped or hour-glass form. They 

 vary from i.'y to 40 /x in height and are conspicuous in cross-sectional 

 view. The spongy parenchyma forms the third layer, consisting 

 of 10 to 1.0 rows of thin-walled cells which are very much com- 

 pressed and crushed at maturity. At the hilum, two layers of 

 palisade cells are present; and, immediately beneath the slit of the 

 hilum, there is a group of sclerenchymatous cells with reticulate 

 walls which, according to Tschirch and Oesterle, may serve a pro- 

 tective function against the invasion of fungi. The subepidermal 

 layer of column cells is expanded beneath the hilum into a cushion 

 in which the porous sclerenchymatous cells are embedded. (Fig. 



174, 5) 



According to Pultz (x5), the seed coat is the only part of the pea 



seed that contains starch during the first eight or ten days of growth 

 following fertilization, but it does not at any time contain enough 

 to affect the edibility of the seed . The development of the palisade 

 layer, which may become very hard, is an important factor in 

 tenderness and succulence. Nucellar tissue is absent in the mature 

 seed, and this is also true of the endosperm, which is entirely ab- 

 sorbed by the growing embryo at the end of 11 or ix days. Prior 

 to this time, the endosperm plays an important part in the nutrition 

 of the developing embryo and contributes to the sweetness and 

 tenderness of the pea until it is more than half grown. 



The epidermal cells of the cotyledons are small, rectangular, 

 arranged end to end in rows, and contain protein and fat but no 

 starch. The subepidermal layer consists of a single row of cells 

 containing some starch; and underlying these are numerous large, 

 thin-walled cells which are more loosely arranged with intercel- 

 lular spaces. The cells of the mesophyll contain starch grains 

 which are variable in shape, being ellipsoid, reniform, or globular 

 with irregular, rounded protuberances. 



Development of the Seedling. — The development of the pea 

 seedling is hypogeal. The imbibition of water results in a soft- 



