174 



PLANT BIOLOGY 



fruits are agreeable as human food, and some of them 

 have been greatly enlarged or " improved " by the arts of 

 the cultivator. The seeds are usually indigestible. 



Burs. — Many seeds and fruits bear spines, hooks, and 

 hairs, which adhere to the coats of animals and to clothing. 

 The burdock has an involucre with hooked scales, contain- 

 ing the fruits inside. The clotbur is also an involucre. 

 Both are compositous plants, allied to thistles, but the 

 whole head, rather than the separate 

 fruits, is transported. In some com- 

 positous fruits the pappus takes the 

 form of hooks and spines, as in the 

 " Spanish bayonets " and " pitch- 

 forks." Fruits of various kinds are 

 known as "stick tights," as of the 

 agrimony and hound's-tongue. Those 

 who walk in the woods in late sum- 

 mer and fall are aware 

 that plants have means 

 of disseminating them- 

 selves (Fig. 252). If it 

 is impossible to iden- 

 tify the burs which one 

 finds on clothing, the seeds may be planted and specimens 

 of the plant may then be grown. 



Suggestions. — 174. What advantage is it to the plant to have 

 , its seeds widely dispersed? 175. What are the leading ways in 

 which fruits and seeds are dispersed? 176. Name some explosive 

 fruits. 177. Describe wind travelers. 178. What seeds are car- 

 ried by birds? 179. Describe some bur with which you are 

 familiar. 180. Are adhesive fruits usually dehiscent or indehis- 

 cent? 181. Do samaras grow on low or tall plants, as a rule? 

 182. Are the cotton fibers on the seed or on the fruit? 183. 

 Name the ways in which the common weeds of your region are 

 disseminated. 184. This lesson will suggest other ways in which 



Fig. 252. — Stealing a Ride. 



