Germination of Forest Trees 13 



The germination of seeds depends principally upon edaphic 

 factors, hence climatic factors will receive little attention here 

 except in so far as they condition the former. It is taken for 

 granted that the morphological and the physiological significance 

 of water, light and heat to plant life are too well known to require 

 discussion here, especially since that phase of botany is funda- 

 mental in all ecological work. 



The water content of the soil is by all odds the most important 

 edaphic factor in determining germination, for while other factors 

 may condition this process to a certain extent, none but water, 

 within certain limits, can prevent it altogether. In a synoptical 

 manner I will briefly call to mind the significance of this master 

 factor in germination and then briefly inquire into the important 

 soil factors and properties that bear directly on the investigations 

 at hand. 



The amount of water in the soil has no direct relation to the 

 amount of water which plants can use. At the outset distinction 

 must be made between the different kinds of water in the soil 

 and which of these are available to plant roots. Usually three 

 kinds of water are distinguished, namely : hygroscopic water, 

 capillary water and free water. Hygroscopic water is that water 

 which plants cannot get owing to the enormous film pressure 

 which holds it. It is also known as the amount of water in an 

 air-dry soil. Capillary water is that water, most of which is 

 available to plants and is held against gravity around the soil 

 particles by capillary forces. Free water is that which is not held 

 either as hygroscopic or as capillary water. It is water influenced 

 in its movements by gravity and is therefore called gravitational 

 or hydrostatic water. Clements (1) calls these echard, chresard, 

 and holard respectively. It will be seen then, that the only water 

 available to plants is a part of the capillary water which surrounds 

 every soil particle and fills every small pore space. 



The principal factors which influence the amount of soil mois- 

 ture available (capillary water) to plants are: 



1. The amount of water reaching the soil. 



2. The catchment of water by the soil. 



3. The water-holding capacity of the soil. 



