How Plants Secure Change of Location 391 



sphere or other rounded body diminishes in size, its bulk, and 

 therefore its weight, diminishes far faster than its surface; or, in 

 other words, the smaller such a body becomes the more surface 

 does it spread in relation to weight. A body has only to reach a 

 certain point of smallness, therefore, when the very slightest air 

 movements are enough to blow it away, and to keep it suspended 

 indefinitely in the air. This is the reason that dust floats as it 

 does; and amongst this dust, and a part of it, float the spores of 

 Bacteria, Molds, Yeasts, Ferns and other spore-bearing plants, 

 which depend on this method for their dissemination. There can 

 be little wonder that such plants are found so widely distributed 

 when we remember how far this dust can be carried by any sum- 

 mer breeze. Among seed-bearing plants, however, the habit of 

 forming a many-celled embryo before separation of the seed 

 from the parent plant, makes the seed too large for this method 

 to be used, though in some Orchids the embryo formation is post- 

 poned, lea\dng the seed small enough, especially when a loose 

 open sac is added, to be transported in this manner. 



The reader who is versed in morphology will observe that I 

 often ignore the distinction between the seed and its accom- 

 panying fruit. From the point of view of the principle and effi- 

 ciency of dissemination, it makes no particular difference whether 

 the disseminating mechanism is formed from a part of the seed 

 itself, or from the associated receptacle, ovary, style, calj'^x, or 

 corolla. And if one asks why a particular plant forms its wing or 

 its plume in this way, and another in that, we can only reply that 

 herein Ues another illustration of the first law of adaptation, — 

 that a new structure when needed is formed from the part which 

 happens to be most available for the purpose, and sometimes that 

 part is one thing and sometimes it is another. Next after the 

 seed itself, however, the disseminating mechanism is most often 

 constructed from the part next contiguous, the ovary; and the 

 frequency of its use for this purpose has caused its retention with 

 the seeds long after the other parts have fallen. It is this per- 



