IOWA ACADEMY OF SCIENCES. 
217 
Fig:, io. Trichomes of thistle leaves and bracts. 
/, from bracts of C. muticus ; 2, leaves of C. discolor ; 3, leaves of C. Iowensis ; 4 , bracts 
of C canescens ; 3, 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. lanceolatus , in a less degree by C. alMssimus and 
C. muticus. 
The involucral bracts are spiny, as in C. lanceolatus r 
somewhat spiny in C. altissimus and C. aroensis with a 
broad glutinous ridge in C. Iowensis , C, discolor and 6b 
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, but many small flying insects, especially 
