540 WEED FLORA OF IOWA 



here of an indistinct layer between the first and second. The 

 layer following this consists of thin-walled parenchyma cells, in 

 some cases considerably elongated but in others short. 



The third layer is followed by the endosperm which consists of 

 a layer of rather thick-walled parenchyma cells. These carry 

 granular protein grains. This is followed by one or more layers 

 of elongated cells, in which the cell cavity is very much reduced. 

 These cells reach their highest development between the folds of 

 the caulicle and cotyledon. 



The cells of the first layer of the embryo are smaller, quite uni- 

 form in size and filled with protein grains and oil. 



Small Peppergrass (Lepidhim apetalum Willd.). 



The cuticle forms a continuous layer over the epidermal cells, 

 which are larger than in L. virginicwm. On the addition of water 

 the cell wall rapidly elongates, emitting a copious mucilage. The 

 cell cavity is very much reduced but longer than in L. virginicwm. 

 It is surrounded by a denser, more or less differentiated part of 

 the cell wall which is more yellow in color than the remainder of 

 the cell wall. The second layer is of a yellow straw color and 

 consists of very minute cells with small cell cavities. 



The cell walls of the third layer are strongly thickened, brown, 

 and serve the same purpose as in the other species. The endosperm 

 consists of thick-walled parenchyma cells. 



In the first layer of cells, the cell walls are very much larger 

 and packed with protein grains. The other layers of endosperm 

 consist of small elongated thick-walled cells with a small cell cavity. 

 These attain their greatest development between the caulicle and 

 cotyledon. In the embryo, the cells of the first row are isodiametric 

 filled with protein grains and oil. The outer cells are elongated 

 larger, and also densely packed with the same material. 



Shepherd's Purse (Capsella bursa-pastoris (L.) Medic). 



The seed coats attain their maximum development in the region 

 of the caulicle. Cuticle covers the epidermal cells, the latter 

 tabular, compressed but in the addition of water elongating, be- 

 coming mucilaginous and showing stratification. 



The second and third layers are brown with thick cell walls. 

 Fourth layer consists of endosperm, one layer of isodiametric cells 

 filled with protein grains, followed by thick-walled cells reaching 



