256 PLANT-LIFE 



able to do so, they would lose by evaporation as rapidly 

 as they gained. In their case evaporation is essential 

 to circulation of moisture in their tissues, and their 

 surface, exposed to the air, is of such a nature that, 

 under normal conditions, evaporation can take place 

 only at an advantageous rate. The very devices, such 

 as cuticles permeated with resin or wax, which lead to 

 retention of water and prevention of undue evaporation, 

 are in land plants inimical to absorption by the general 

 surface. In brief, the land plant of the normal type is 

 dependent for its water-supply upon such as is held by 

 the soil, and it absorbs all it requires by means of a 

 special feature of its structure — its root hairs. Soil has 

 a considerable capacity of holding water by capillarity, 

 which prevents it from draining ofi. Sachs states that 

 sand in this way retains 21 per cent, of water, loam 

 52 per cent., and ordinary cultivated soil 46 per cent. 



We understand that what in Seaweeds is often mis- 

 taken by the casual observer for a root is merely a hold- 

 fast. It does not absorb food material from the rock 

 to which it is attached. We may also assume that the 

 root of a land plant is a holdfast, a means of fixity in 

 the soil, but it is a holdfast which has formed the habit 

 of sending out filaments of protoplasmic cells into the 

 water-containing soil. Among the water plants which 

 originally set out to conquer the land and colonize it, 

 those which managed to form a vital connection with 

 the soil by means of root-hairs, as these filaments are 

 called, were destined to be the forerunners of highly 

 complex and successful land plants. For in these the 

 development of the root-hair, which can explore the 

 soil in quest of moisture, and absorb it through its cell 



