28 METABOLISM 



the formation of a root-system distributed widely in an almost horizontal 

 direction ; the tap-root, however, still remains in existence. The tap-root 

 of the spruce certainly penetrates the soil at first deeply, but after five years or 

 so it ceases to grow, so that the tree in later stages of development is quite 

 superficially rooted. The fir alone becomes a deeply-rooted tree with a pre- 

 dominant tap-root. An example may now be taken from deciduous trees. 

 The beech, according to Hartig (as cited by C. Kraus, 1892), bears in its first 

 year a simple tap-root with a few lateral branches. By the third year the 

 most superficial of these take on vigorous growth and develop into a richly 

 branched root-system close to the surface of the soil. By the fifth or sixth 

 year the tap-root, which has now attained a length of at most \ m., ceases to 

 grow, and only the lateral roots go on developing. Up to an age of thirty 

 years, two or, more rarely, three of the deeper seated lateral roots develop pre- 

 eminently, pushing their way obliquely into the deeper layers of the soil. After 

 that, onwards, the more superficial roots overtake them as far as rate of growth 

 is concerned, and spread themselves out horizontally beneath the surface of 

 the soil, constituting the chief part of the root-system. Hence when the tree 

 is felled the root-system is found to form an unusually shallow layer in com- 

 parison with its horizontal extension, being at most 60 cm. deep. 



Since the time of Hales (1748) many estimates have been made as to the 

 extent of the root-system of different plants as well as with regard to the amount 

 of soil laid under contribution by them. Thus Nobbe (1872) has shown that 

 the aggregate length of all the roots of a one-year-old wheat plant amounts 

 to 5oo-i6oo m. ; that of a pumpkin may reach 25 km. (Sachs, 1882, p. 19), while 

 Schumacher (1867) has made measurements by weight of the root-system of 

 several cultivated plants. Sachs (1882, p. 19) estimated the space occupied by the 

 roots of a sunflower at one cubic metre, so that one may safely conclude that 

 the root-system of a large tree is distributed through hundreds of cubic metres 

 of soil. Inquiries of this kind, however, are, from the physiological point of view, 

 not of much service since, as is well known, all roots have not the same functions. 

 In perennial root-systems we may distinguish conducting and absorbing roots. 

 The former are the permanent parts of the root-system, soon becoming covered 

 with cork on their outer surfaces and taking no further part in the absorption 

 of water ; they serve to fix the plant firmly in the soil, however, and also to 

 carry the absorbent roots. The latter are thin and remain so, and after 

 a certain time disappear. These are the roots which absorb the water, although 

 by no means their entire surface subserves this purpose — only that of the 

 extreme apices where these are covered with hairs, or where hairs have not yet 

 developed (Kny, 1898). Roothairs are always wanting in some land plants, and 

 the general epidermis performs the function of water absorption in such cases. 

 Apart from such exceptional conditions we may designate the roothairs as the 

 special organs for the absorption of water. The roothairs are tubular pro- 

 longations of epidermal cells, sometimes of considerable length, which have the 

 effect of increasing very considerably the absorbent surface of the root. 

 F. ScHWARZ (1883) has reckoned that the surface of the root of the maize is 

 increased five and a half times by the formation of roothairs, of barley, twelve 

 times, and of Scindapsus, eighteen times. New roothairs are developed acro- 

 petally from day to day on the elongating root as the older hairs die off behind — 

 for the roothairs live for only a short period. Regions covered by dead hairs 

 absorb water only with difficulty, so that we must calculate the area of the 

 apical regions as well as the increase of surface due to roothairs, if we are to 

 arrive at a correct estimate of the region of functional activity of the root. 

 Such estimates have not as yet been made. 



If we now inquire how the individual roothair absorbs water from the 

 soil it will aid us considerably if we study carefully Sachs's statement on the 

 subject (1865) taken in conjunction with an examination of Fig. 5. This 



