MICROSCOPIC STRUCTURE OP WEED SEEDS 543 



The third layer consists of thick-walled parenchyma cells, densely 

 packed, radially elongated, sides presenting a cone-shaped appear- 

 ance. Underneath this is a layer of thick-walled parenchyma cells 

 which contain some coloring matter. The endosperm follows this 

 layer. The first layer consists of thick-walled cells, densely packed 

 with albuminous matter. The remaining cells vary in number, 

 are much elongated, thick-walled with a small cavity; these cells 

 extend down between the contiguous portions of the cotyledon or 

 caulicle. 



The Embryo. — The cells of the first layer surrounding the 

 cotyledon or caulicle are smaller, filled with fat and protein grains. 

 The remaining cells are larger, also filled with fat and protein 

 grains. The central part of the caulicle shows a differentiation of 

 the embryonic vascular portion, consisting of small cells. 



Charlock or Common Mustard (Brassica arvensis (L.) Ktze.). 



The outer layer of cells is compressed, tabular, with stratification 

 not evident, and cuticle well developed, and forms a continuous 

 layer over the outer cells; on the addition of water, the cell walls 

 become mucilaginous, elongate, stratification becomes evident, the 

 cuticle breaks, and an irregular surface is formed. The second 

 layer is but slightly developed, made up of thin-walled parenchyma 

 cells. The cells of the third layer are elongated and thickened 

 laterally. These cells are much longer than in B. nigra and brown 

 in color. The fourth layer consists of one to two rows of rather 

 thin-walled cells carrying pigment. Endosperm consists of several 

 rows of cells; first row nearly isodiametric, filled with protein 

 grains. The three or four layers of cells following are thick-walled 

 with a small cell cavity. 



Embryo. — First layer of cells nearly isodiametric, those follow- 

 ing somewhat larger, filled with protein and fat grains. 



Hedge Mustard {Sisymbrium officinale Scop.). 



Cuticle covering the epidermal cells, the latter tabular, much 

 compressed. On the addition of water the cell walls become 

 mucilaginous with evident stratification. The cells of the second 

 layer are brown and thin-walled, much compressed. On addition 

 of choloral hydrate they expand. Third layer much darker than 

 the second, thick-walled, followed by endosperm, cells elongated, 

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



