156 LIFE : OUTLINKS OF GENERAL BIOLOGY 



arc no true r(X)ts comparable to those of higher forms. What look 

 like roots are anchoring structures (rhizoids), and this must be 

 regarded as the primary function, absorption being secondary. 

 Similarly, lichens are firmly fixed to the rocks and the like, some- 

 times on the mountain-tops, by means of hold-fast "rhizoids", very 

 like root-hairs in apjx^arance, though there are no true absorbent 

 roots. They arc also to be seen in liverworts, mosses, and the pro- 

 thallia of ferns — delicate, threadlike outgrowths of surface-cells, more 

 for attachment than for absorption. In ferns there is a first appear- 

 ance of true roots like those of seed-plants; and the line of evolution 

 has been the addition of an absorptive to an anchoring function. 



Among the many adaptations of roots to particular conditions, 

 the following may be noted. Plants frequenting dunes, where the 

 wind often shifts the sand, may show roots of great length, often 

 extending for yards, and sometimes branching into a network. 

 From these, at intervals, shoots rise above the surface. Good examples 

 are the sand-binding grasses {Elymus arcnaria and Ammophila 

 arenaria) and the sand-binding sedge [Carex arcnaria), which secure 

 their own survival by gripping the shifting sand over a wide area, 

 and are also of great importance to man in preventing sandstorms 

 which are apt to smother arable land. 



On many tropical shores the substratum is loose and swampy, 

 and more or less flooded by high tides and storms. This difficulty is 

 met in species of Pandanus (so-called screw-palms) by numerous 

 stilt-like roots, which are given off from the stem at various levels 

 and sink down into the loose sand and mud like so many anchors, 

 some descending vertically and others obliquely or in an arch. This 

 circumvention of a difficulty reaches perfection in the Mangroves 

 (mostly sjx?cies of Rhizophora), where the stilt-roots are very 

 numerous and often very long, forming extraordinary root-thickets. 

 Those trees that go farthest out are most abundantly productive 

 of the.se adventitious roots, and they form a sort of breakwater to 

 those nearer the firm shore. Schimper notes in regard to a common 

 sj)ecies of mangrove, Rhizophora mucronata, that the stilt-roots are 

 not develojxKl when the tree is cultivated on dry ground. That is to 

 say, the power of forming these adventitious growths is a specific 

 character, but it requires an appropriate nurtural stimulus if it is 

 to be expres.sed. 



The cases of dune-plants and mangroves must serve to illustrate 

 the adaptation of roots to meet peculiar conditions of the sub- 

 stratum, but there are many others. Thus many fig-trees in humid 

 equatorial forests (such as Ficus elustica, common in our green- 

 houses) have extensive superficial roots which spread round the base 

 of the stem like narrow vertical planks, and resist the strain on the 

 tree, which is often, though not always, slender and high. 



Inadequate Food-materials.— Soils differ greatly in chemical 



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