CH. VII, 3] DISSEMINATION OF PLANTS 357 



need new host plants; so that plants, in a manner like 

 animals, must spread in search of food. Accordingly, dis- 

 semination is not a mere incident of plant life, but a neces- 

 sary function. 



The general methods of plant dissemination and dispersal 

 fall under some six heads as follows : 



1. Independent locomotion. While none of the familiar 

 land plants have any such power, it is frequent in the lower 

 Algae. Thus, as will appear more fully in Part II of this 

 book, some of the simplest kinds work their way over the 

 bottom of ponds by aid of protoplasmic threads, while others 

 jerk their bodies along by sudden vibrations. More famil- 

 iar and typical, however, are the ZOOSPORES possessed by 

 many Algae (page 301, Fig. 213), which can swim by action 

 of vibratory cilia in a manner so animal-like as to have 

 originated their name. The Slime Molds, or Myxomy- 

 cetes (page 38, Fig. 14), living out of the water but in damp 

 places, can creep over wet surfaces in a manner identical in 

 aspect and method with that of the animal Amoeba. 



2. Extension through growth. As described in earlier sec- 

 tions (pages 187-9), many plants send off stems along the sur- 

 face of the ground or beneath it, in the forms called stolons, 

 offsets, runners, and rootstocks, which take root at their 

 tips and there form new plants, after which the old con- 

 nection with the parent often withers away. The Straw- 

 berry offers a familiar and typical example of this mode 

 of spread ; the suckers which spring from the ground in the 

 vicinity of fruit trees, often from old roots, are other ex- 

 amples ; but it reaches perfection in the Grasses, especially 

 the familiar Couch Grass of the gardens. Of course all 

 such plants have likewise a dissemination by their seeds, 

 their spread by growth-extension being additional and often 

 incidental. In fact all gradations in this method are 

 found from cases clearly incidental or accidental up to 

 those which seem clearly adaptational, in which fact is prob- 

 ably embodied a leading principle of evolution, viz. that 



