578 



WEED FLORA OF IOWA 



as thick-walled as the epidermal, pigment yellowish. The elong- 

 ated sclerotic parenchyma cells are yellowish white or nearly color- 

 less ; this layer is followed by the parenchyma cells of the testa and 

 remnants of the nucellus. The parenchyma cells of the embryo 

 with numerous small intercellular spaces contain protein and fat. 

 The fibrovascular bundles are located one at each end of the achene. 



Fig. 440. Microscopic structure of the seeds of Thistles (Cirsium). 

 I. and II. Common Wood Thistle (Cirsium discolor). III. Iowa Thistle (.Cir- 

 sium ioense). 

 ep=thick-walled epidermal cells. pi=pigment layer. scl=sclerotic palisade cells 

 with narrow cell-cavity. p=parenchyma cells of testa. t=testa. n=nucellus. 

 em=embryo. fv=fibro-vascular bundles. 



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



Iowa Thistle (Cirsium ioense (Pammel) Fernald). 



The microscopic structure of the seeds is similar to that of pre- 

 ceding, comprising a thick-walled epidermal layer with bright 

 colored walls and underlying pigment layer, an elongated sclerotic 

 parenchyma and the testa of small parenchyma cells. 



