752 RELATION OF STRUCTURE TO FUNCTION IN ROOTS. 



is temporarily interrupted by drought or cold, frequently have a third function 

 to perform, viz. that of storing up starch, fat, sugar, and other reserve food- 

 materials. Obviously the parts concealed in the ground are protected in a high 

 degree against aridity and frost in countries with long-continued summer drought 

 or with severe winters, and therefore the underground root-structures principally, 

 together with underground parts of stems and the scale-leaves arising from them, 

 can be used most advantageously as reservoirs for the materials formed in the 

 green organs above ground during the short period of vegetation. 



The variety of functions assigned to roots, the diversity of the substrata, and 

 the peculiar conditions of the habitat and climate render necessary a large number 

 of different forms, the most noticeable of which bear special names in botanical 

 terminology, and will be briefly enumerated here. According to the substratum 

 into which the roots penetrate, and from which they derive water and food, we 

 may distinguish between subterranean, aquatic, aerial, and parasitic roots. 



Subterranean roots {radices hypogaeoe) push their ends, which are beset with 

 root-hairs, into the ground with great energy, and are entirely covered over with 

 soil, or at any rate in so far as their absorbent portions are concerned. Roots 

 proceeding from the radicle of the embryo are chiefly subterranean. The roots 

 springing from the different forms of scaly stem are almost all subterranean, and 

 we shall not be far wrong if we estimate the roots of 70 per cent of all existing 

 phanerogams as subterranean. 



Aquatic or floating roots (radices natantes) spring laterally from floating stems 

 and are generally arranged in clusters, more rarely singly, and are to a slight 

 extent spirally twisted. They are developed both from stems whose foliage-leaves 

 lie flat on the surface of the water, and also from the floating, leafless stem- 

 structures which have been metamorphosed into phylloclades (e.g. in Lemna polyr- 

 rhiza, gibba, Tninor). In these plants the root-tips are also surrounded by water. 

 If they come to lie on the slimy bottom in consequence of a fall in the water- 

 level, they do not penetrate into it, nor do they enter into relations with the 

 particles of mud. Marsh-plants, on the contrary, send their first roots right down 

 into the mud, whilst those developed subsequently from the higher internodes 

 are allowed to float in the water. The primary root produced from the seed 

 of the Water Soldier (Stratiotes aloides) is embedded in mud, and is therefore 

 really a subterranean root; after it has died off the whole plant rises up, remains 

 oscillating below the surface of the water, and develops floating roots from its 

 abbreviated, leafy stem; later the plant again sinks down, and the floating roots 

 again become subterranean (cf. the account on p. 76). Conversely it often 

 happens that subterranean roots become transformed into aquatic roots. In 

 alders, willows, and elms growing on the sides of streams, extensive net-works 

 of roots are often to be seen which have grown out from the slope of the bank 

 and float in the water. Indeed, many terrestrial roots, strangely enough, exhibit 

 a much more luxuriant growth in flowing water than in the ground, and it is 

 well known that the roots of the above-named trees, when they have effected an 



