PHYSIOLOGY 197 



for the nutrient water taken up by the roots is so weak in mineral 

 substances (it contains but little more solid matter than good drinking- 

 water), that the plant would obtain too little food if it were only able 

 to take up as much water as it could retain and make use of. 



ALL THOSE CONTRIVANCES, IN PLANTS, THEREFORE, WHICH RENDER 

 POSSIBLE OR PROMOTE EVAPORATION, OPERATE CHIEFLY IN THE SERVICE 



OF NUTRITION. Were transpiration not in the highest degree useful 

 and even necessary for the acquisition of mineral substances, provision 

 would certainly have been made by plants to restrict it within the 

 narrowest possible limits. For transpiration increases the amount of 

 water required by plants disproportionally to their powers of absorp- 

 tion, and exposes them, moreover, to the danger of perishing through 

 the insufficiency of their water-supply. In spite of the increased 

 danger of drying up, as the result of evaporation, special provision is 

 made by plants for facilitating transpiration (p. 205). 



The Absorption of Water. The "water" absorbed by the plant 

 is not chemically pure water, but a DILUTE WATERY SOLUTION OF 



VARIOUS SUBSTANCES FROM THE SOIL AND FROM THE ATMOSPHERE. 



Through the peculiar selective power of their cells (p. 194) plants 

 exercise a choice from among the substances available in the nutrient 

 solution. 



Aquatic plants and the lower land-plants which are but little 

 differentiated, such as the Mosses, can absorb water by their general 

 surface. The same is true of many aquatic Phanerogams. These 

 (Utricularia, Ceratophyllum, Wolffia), like some Hymenophyllaceae of 

 damp primaeval forests, the epiphytic Tillandsia usneoides and the Sun- 

 dew, possess as a rule no roots. The roots of many submerged plants, 

 on the other hand, penetrate the soil and contribute essentially to the 

 nutrition of the plant. 



In plants living on dry land the conditions are quite different ; 

 their stems and leaves develop in the air, and they are restricted to 

 the water held by capillarity in the soil. In order to obtain this 

 water in sufficient quantities, special organs are necessary, which may 

 spread themselves out in the soil, and enter into intimate connection 

 with its particles, in their search for water. These organs must 

 absorb the water from the soil, and then force it to the aerial 

 portions of the plant. This office is performed for a land plant by 

 its root system, which, in addition to providing the supply of water, 

 has also the task of mechanically fastening the plant in the soil (~). 



Conversely, loose soil is naturally bound together by the branching roots ; and 

 on this account plants 'have an economic value in holding together loose earth, 

 particularly on dykes and land subject to inundation. 



The small clod of earth, that is as a rule at the disposal of a plant 

 in its natural habitat, is utilised to the full by the highly developed 

 root system, which behaves in a wonderfully purposive manner. 



