366 STRUGGLE BETWEEN PLANT-COMMUNITIES sect, xvii 



CHAPTER XCVIII. THE WEAPONS OF SPECIES 



There is scarcely any biological task more attractive than that of 

 determining the nature of the weapons by which plants oust each other 

 from habitats. Yet we are far from having exhaustively solved the 

 problem even with regard to a single species ; for instance, we do not 

 completely comprehend the struggles between the beech and oak, or 

 between other economically important forest-trees. Obviously the matter 

 is not settled by asserting that lack of available space is decisive, or that 

 in the plant-world, as in all other communities (including the human race), 

 everything turns on the question of nutrition. Such statements scientifi- 

 cally analysed resolve themselves into a series of the most difficult ques- 

 tions which science could propound, and which could be answered only 

 after many-sided investigations. For instance, there arise such questions 

 as : 'Is it lack of one or another nutritive body or of water in the soil ? 

 Or the excess of another substance ? Is it want of heat or of light or of 

 an appropriate combination of these ? Or can roots and rhizomes grow 

 so close together as to bar the way to other plants in a purely mechanical 

 manner, or so as to rob them of water and nutriment ? ' ^ And so forth. 



We see perennial herbs extinguishing annuals that have settled on 

 ground which was bare but a short time before ; but with what weapons 

 the former conquer we cannot say with any certainty. We see silicicolous 

 vegetation of sand (Ornithopus perpusillus, Teesdalia, Spergula, Rumex 

 Acetosella, Pteris aquilina, and others) disappear when the sterile field is 

 supplied with lime (either by special addition of lime as a food-material, 

 or by a change in the lime already present so that this becomes more 

 easily available to plants) ; and we see this vegetation gradually return 

 as the carbonic acid in the water dissolves and carries away the lime ; 

 but we can give no deeper explanation of these phenomena. 



Living beings forming a community have their hves linked and inter- 

 woven into one common existence in so manifold, intricate, and complex 

 a manner that change at one point may bring in its wake far-reaching 

 changes at other points. In this direction a wide field lies open for 

 investigators. 



To Stebler and Volkart ^ we owe an interesting piece of work on the 

 influence of shade on the distribution oi plants. 



Not only do the manifold relationships shown by species to the oeco- 

 logical factors, light, water, heat, and so forth,^ play a great part in these 

 changes, but so likewise do the different biological characters of growth- 

 forms;* and of these characters we cannot say that they are the direct 

 result of the factors above mentioned. If forest becomes the final stage 

 of the vegetation in a whole series of habitats, this result is due, inter alia, 

 to the longevity and large size of forest-trees, which can raise themselves 

 above, and cast their shade over, herbs and shrubs, and can produce 

 numerous seeds year after year. In this way forest-trees easily gain the 

 mastery over many other growth-forms, even if only a solitary individual 

 originally succeeded in gaining entrance to the community. In such 



' Concerning the struggles between roots of trees, see Fricke, 1904. 



* Stebler und Volkart, 1904. ^ See Section I. * See Chap. II. 



