IOWA ACADEMY OF SCIENCES. 



217 



Fip. 10. Trichomes of thistle leaves and bracts. 



/, from bracts of C. muticus ; 2, leaves of C. discolor; j, leaves of C. lou'ensis : 4, bracts- 



of C cancscens ; 5, leaves of C. lanceolatus. 



Protection. — The thistles are admirably protected from 

 herbivorous animals by their spiny leaves and bracts. 

 This is true in a marked degree by such species as C. arven- 

 sis and C. lanceolatiis, in a less degree by ('. ultissinms and 

 C muficus. 



The involucral bracts are spiny, as in C. /anceolatHS^ 

 somewhat spiny in ('. (i/fi.^siiHHs and C. (n-rcnsls^ with a 

 broad glutinous ridge in C. loivensis, C. discolor and C. 

 Hillii The spiny bracts serve to protect the plant from 

 herbivorous animals and some crawling insects. This glu- 

 tinous ridge not only prevents crawling insects from going 

 into the head, bat many small flying insects, especially 



