46 OECOLOGICAL FACTORS AND THEIR ACTION SECT, i 



the level of ground-water ; he gives, for instance, 1 one as showing how the 

 vegetation of a district gradually changes with the descent of the water- 

 table. Many trees assume a peculiar shape, or cannot grow at all on soil 

 with ground-water near the surface. Warming 2 gives additional exam- 

 ples ; but in these cases further investigation is required to demonstrate 

 what part is played by the level of ground-water, and what by other 

 properties of the soil, including its power of raising water. 



In addition, periodic fluctuations in the level of ground-water embracing 

 a number of years (Bruckner's thirty-five year periods) has been recog- 

 nized. These observed fluctuations of climate cannot be associated with 

 Blytt's theory of alternating moist and dry epochs, and corresponding 

 changes in the vegetation. 



In the layer of soil lying above the water-table the amount of water is 

 influenced by the following important characters : facility of percolation 

 in soil, hygroscopic character of soil, its power of raising water, its water- 

 capacity, as well as the amount of atmospheric precipitations 3 and the 

 influx of surface-water. 



Facility of percolation in the soil. Atmospheric precipitations do not 

 penetrate all kinds of soil with equal facility, as may readily be seen 

 if water be poured on sand, clay, and humus. The following factors 

 play a part in this matter : the water-capacity of soil, also the kind and 

 dryness of the particles of soil. 



The greater the water-capacity the more slowly does water sink in 

 the soil. Very fine-grained soil, especially clay and certain humus- 

 soils, are almost impermeable to atmospheric precipitations when the 

 particles are densely packed ; whereas the more coarse-grained and loose 

 is the soil the more freely is it penetrated by atmospheric precipitations. 

 If the soil be rich in good-sized stones or in crevices and cavities, such 

 as the burrows of earthworms, then the velocity of penetration is 

 modified by these : it is decreased by stones, but increased by crevices 

 and cavities. 



Water penetrates most readily into quartz-sand, less readily into 

 humus, and least fully into clay. Clay soil permits the percolation of 

 water with difficulty, not only because of the small size of its particles, 

 but also because of their other characters. 



If the uppermost layers of soil be very dry, some time elapses before 

 they are so wetted as to allow the infiltration of water to commence. 



The hygroscopic character of soil. All porous and dry soil can absorb 

 aqueous vapour, though to a very varied extent. The absorbed water 

 vapour is invariably available to the plant, because it is taken in only 

 when the earth is dry ; it can never provide too much water. On the other 

 hand it is not capable of alone supplying dry soil with water sufficient 

 for the needs of plants ; these wither before the amount of water in the 

 soil has decreased to such a degree that absorption of aqueous vapour 

 takes place. 



Power of soil to raise water. The power that soil possesses of raising 



water from the deeper layers is obviously of importance to plant life. 



But we must distinguish between the heights to which, and the velocity 



with which, water is raised. These depend, inter alia, upon capillarity 



1 Feilberg, 1891, p. 270. * Warming, 1887, 1890, 1891. 



3 See Chap. VII. 



