MICROSCOPIC STRUCTURE OF WEED SEEDS 517 



are conspicuous, not only because of their size, but because of their 

 yellow or brown color, b. Endosperm. 1. Aleurone Layer (al). 

 The individual cells of this layer are characterized by their 

 great variation in size (0.01 to 0.04 mm. in diameter) and form. 

 2. Starch-Cells (st). In the outer layers the starch granules, if 

 present, are much smaller than in the interior of the seed, where 

 they sometimes reach a diameter of 0.03 mm. They are usually 

 sharply polygonal, with a distinct hilum and radiating fissures. 

 The starch is surrounded by small protein granules, forming a 

 network which is especially evident after removing the starch by 

 reagents. In some specimens, one or more of the outer cell layers 

 are filled with these protein granules to the complete exclusion of 

 the starch. 



Sandbur {Cenchrus tribuloides L.). 



The pericarp is divided into two portions. The cells of the 

 outer portion are thick-walled and short; of the inner portion, 

 elongated, thick-walled and fusiform. The testa consists of one to 

 two rows of thin-walled cells, much compressed. The aleurone 

 cells are much larger than in Panicwn and Setaria, thick-walled, 

 densely filled with protein grains. Walls of the starch cells thick- 

 ened; starch grains larger and loosely arranged. 



Timothy (Phleum pratense L.). 



The testa and pericarp are dark colored. The epidermal cells 

 are thin-walled, elongated, sometimes slightly irregular. The 

 testa consists of several rows of thick-walled dark brown cells 

 much longer than broad. The aleurone layer consists of a single 

 row of cells relatively thin-walled, somewhat variable in size, 

 solidly packed with aleurone grains. The nucellus is very evident 

 as a remnant in some places. The cells of this layer are thick- 

 walled, clear and colorless. The starch cells are much larger than 

 the aleurone, and contain angular starch grains. 



Cheat or Chess (Bromus secalinus L.). 



Dr. A. L. Winton describes the microscopic structure as follows: 

 Flowering Glume. — The structure throughout is much the same 

 as in darnel, but the cells of the outer epidermis are much more 

 conspicuously thick-walled, and the wavy-walled cells throughout 

 much longer than broad. The circular cells also have wavy walls. 

 The cells on the margins, interspersed with lance-shaped hairs, are 

 the same as in darnel. 



