MICROSCOPIC STRUCTURE OP WEED SEEDS 



525 



pressed nucellus follows. The endosperm consists mostly of a 

 single layer of cells, the embryo of small epidermal cells and the 

 elongated palisade parenchyma on the upper face of the cotyledon ; 

 cells of embryo and endosperm contain fat and protein. 



Nettle (Urtica gracilis Ait.). 



The pericarp of the small achenes consists of an outer epidermal 

 layer of rather large cells with exterior walls thickened, and 

 underneath several layers of loosely arranged parenchyma cells. 

 The testa and nucellus are compressed ; cells of the embryo squarish, 

 containing fat and protein grains. 



33QQKs^^Z3« 



Fig. 403. Microscopic structure of the seeds of the Nettle family. (Urticaoeae). 



I. Hemp {Cannabis sativa). II. Common nettle (Urtica gracilis). 

 ep=epidermis, thick-walled short cells, underneath followed by a pigment layer, 



thinner- walled parenchyma cells and a layer of thick-walled cells. pal= 



palisade cells. p=parenchyma cells with the pigment. t=testa. n= 



nucellus. em=embryo. 



(Drawings by L. H. Pammel and Charlotte M. King.) 



POLYGONACEAE,* BUCKWHEAT FAMILY. 



Miss E. Sirrine finds, "In the seed coats of the family, the 

 palisade portion constitutes the outer part of the achenium ; this is 

 followed in most cases by the testa consisting of several layers of 

 cells varying, however, in some cases; they are quite regular in 

 form and in some instances are of a dark color. In the mature 

 palisade cell, the cavity is present; this varies greatly in the dif- 

 ferent genera; in some cases occupying nearly the entire cell, in 

 others small and irregular." 



*A study of the microscopic structure of the achenes is largely based on Miss 

 Emma Sirrine's work and upon that of Dr. Winton. 



