214 



Introduction to Botany. 



Fig. 130. 



Seed pods of the Cottonwood 

 just before breaking open. 

 Reduced. 



where the hairs are an outgrowth 

 from the base of the seed, the total 

 weight of the seed and hairs is 1.5 

 miUigrams ; while in the seed pod 

 the hairs are closely appressed 

 against the seed, but after the pod 

 breaks open the hairs begin to dry 

 and bend downward and outward, 

 and in so doing they assume the 

 form of a parachute and spread the 

 seeds apart at the same time, so 

 that the slightest puff of wind car- 

 ries them away. On a single tree 

 the pods may be breaking open in 

 succession and offering their seeds 

 to the wind for a space of two 

 months, so that the seeds are quite 



certain to be borne away 

 by winds from all direc- 

 tions. In strong winds, 

 such seeds must be car- 

 ried to very great dis- 

 tances (Fig. 131). 



In the tumbleweeds 

 another method has been 

 devised for employing 

 the wind in scattering 

 the seeds. These plants 

 have a rounded general 

 contour; when the seeds 

 have ripened, the whole 

 plant dies and breaks off 

 close to the ground, and 

 is easily rolled by the 



Fig. 131. 



On the left seed pods of Cottonwood breaking 

 open and the hairs on the seeds beginning to 

 spread apart and push the seeds out of the 

 pods ; on the right, a later stage, the seeds 

 ready to be wafted away by the wind. Reduced. 



