574 



WEED FLORA OF IOWA 



Fig. 438. Microscopic structure of seeds of some cultivated weeds. 

 I. Wild Cucumber or Wild Balsam Apple (Echinocystis lobata). II. Bur 



Cucumber (Sicyos angulatus). 

 ep=epidermis. ll=light line. p=parenchyma. m=malpighian layer. scl=scler- 



otic cells. 

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



COMPOSITAE, COMPOSITE FAMILY. 



Large Ragweed {Ambrosia trifida L.). 



The so-called "seed" consists of an involucre of rather thick- 

 walled sclerotic parenchyma cells, occurring underneath the 

 epidermis; some of these cells are radially elongated, others are 

 spherical in cross section, showing numerous pore canals. The 

 internal layer of the involucre is composed of nearly isodiametric, 

 thick-walled, sclerotic cells. The testa consists of a layer of brown- 

 ish colored cells followed by a layer containing black pigment. 

 Next within lies a layer of thick-walled, small, nearly colorless 

 cells, then the compressed layer of the nucellus, followed by the 

 nearly square cells of the embryo. 



