26 



METABOLISM 



that there are very few land plants which are capable of developing normally 

 when their root-systems are submerged in such water-culture solutions (Lect. VII), 

 for under these conditions the only oxygen available is that dissolved in the 

 water — an amount far below that present in a well-aerated soil. In water- 

 cultures, moreover, the root is always able to find some free oxygen, whilst in 

 swampy situations the minutest traces of this gas are often used up. Swamp 

 plants overcome this difficulty by providing themselves with an elaborate 

 intercellular space-system by means of which the gas may enter the root from 

 above, and frequently by the formation of special respiratory roots which pro- 

 ject above the medium (Goebel, 1886, 1887 ; Jost, 1887 ; Karsten, 1892). 



Let us now consider the case where the subsoil is permeable to water. The 

 water soaking into the superficial layers of the soil will partly, at least, drain 

 into the deeper layers and carry the air with it into the larger interspaces, 

 but all of it does not percolate downwards in this way. Some remains adherent 

 to the individual soil particles, while more collects in the minute cracks and 

 cavities in the particles and is firmly held there by capillarity. The amount 

 of water which remains in the soil as compared with the volume of the soil 

 is termed the soil's water capacity. Water capacity varies with the nature 

 of the soil, and also, and chiefly, with the number and size of the spaces existing 

 between the soil particles. This variation, within wide limits, is very con- 

 siderable. The following data will serve to illustrate this fact : — 



Water capacity of different soils 

 (Ad. Mayer, 1901). 



Humus soil 

 Clay soil 

 Fine sandy soil 

 Coarse „ 



Vol. % 

 . 55 



• 53 



• 30 

 . 10 



Water capacity of quartz soils 

 (WoLLNY in Ramann, 1893, p. 67). 



Vol. % 



Size of grain i.oo-2-oo mm. 3-66 



„ „ 0-25-0.50 4-38 



,, o- 1 1 -0-17 603 



„ 0-0I-0.07 3550 





These amounts of water are retained by soils only immediately after 

 a thorough soaking ; part is lost again in the process of evaporation. Plants, 

 however, during their vegetative period, when most water is required, are 

 frequently obliged to obtain it from a relatively dry soil, and hence they must 

 be provided with a greatly branched root-system with the utmost possible 

 absorbent surface. 



The importance of the root for the purpose of water absorption is evidenced 

 by the fact that in the seedling the root is driven into the soil long before the 

 leaves unfold. In a word, water is what the seedling primarily needs, since 

 all the other substances required by it are supplied in abundance by the reserves 

 stored up in the cotyledons or in the endosperm. In many cases the primary 

 root derived from the radicle of the seedling remains active for many years, 

 or, it may be, for life ; it grows in length and may pierce the soil to a very 

 considerable depth, should the nature of the deeper layers permit. In desert 

 plants especially, tap-roots of enormous length may be developed, which are 

 primarily of service in the absorption of water from the deeper layers of the 

 soil. As a general rule, however, the primary root is not the only active 

 agent ; it is aided by a series of lateral roots, which arise from it in acropetal 

 succession, sometimes almost at right angles from the parent root and pene- 

 trating the soil horizontally, or curving downwards at angles of 70°, 60°, or 50°. 

 In many plants, e. g. in Vicia faha, according to Hellriegel (1883), the main 

 root shows continued growth after the appearance of the lateral roots, whose de- 

 grees of development are approximately indicative of their age, those nearest the 

 apex being the shortest, those farthest away the longest, whilst the apices of all 

 of them lie approximately on the surface of an imaginary cone, whose apex is 

 the tip of the main root itself. Another type of root-system is exemplified by 

 the yellow lupin. Here the lateral roots are much fewer in number and more 



