184 PLANT COMMUNITIES 



The seeds of the violet and of the hipine are thus hurled out to a dis- 

 tance of several decimeters and those of the witch hazel are often 

 scattered for several meters. In the Geranium the carpels separate 

 from the base of the main axis of the fruit and curl upward with such 

 suddenness as to discharge the seeds. In the touch-me-not {Im- 

 patiens) the mature fruit is under such tension that a mere touch 

 causes it to dehisce violently and so scatter the seeds. The fruits 

 of the dwarf mistletoe (Arceuthobium) explode at maturity and hurl 

 the seeds for several meters, and, since the seeds are sticky, they 

 readily adhere to the bark of trees on which the mistletoe is parasitic. 



Many capsules and pods do not dehisce violently but open up and 

 remain in such a position that their seeds are easily shaken out by 

 wind or brushed out by animals. In many mints the nutlets may be 

 shot out if the calyx is pressed down and then released, and in the 

 smartweed {Polygonum virginianum) the akene is attached to an 

 elastic cushion in such a way that when it is pressed back and then 

 released it bounds off to a distance of two or three meters. These 

 means of dissemination are of course relatively inefficient because 

 the seeds are never scattered more than a few meters from the parent 

 plant. 



The spores of many fungi are forcibly discharged at maturity. 

 The fungus Pilobolus grows only on dung piles and when the spores 

 are mature they are discharged in masses which usually fall on the 

 grass or other vegetation surrounding the dung pile. They can never 

 germinate and grow on the grass but if the grass is eaten by an ani- 

 mal the spores pass through the alimentary canal and are deposited 

 in a suitable substratum for germination and growth. The spores of 

 many Ascomycetes and Basidiomycetes are discharged into the air 

 and are then caught by air currents and carried away. 



Many plants with rhizomes or runners depend more upon growth 

 than upon the dissemination of seeds or fruits for migration. Migra- 

 tion by growth is a very slow but otherwise efficient means of ad- 

 vance into new territory. 



IIG. Migration Through the Aid of Wind.— Wind is probably the 

 most efficient of disseminating agents, at least as to the numbers of 

 seeds or spores carried. Spores being lighter than seeds are often 

 carried much further, though very small seeds, such as those of 

 orchids, may be carried for great distances. 



Many adaptations for wind dissemination are found. In the 

 maple (Acer), the hop tree {Ptelea), and the elm ( Ulmus) the fruits. 



