THE PHYSIOLOGICAL ROLE OF WATER IN PLANTS. 11 



We think the time has iirrived to study with gTeatcr care the abso- 

 liiU' Aahie of the different oi)ti]iia wliieh are recojiiiized in l)iok)<;y. It 

 is well known that there are optima of temperature, of light, of plant 

 food, and of humidity with which to realize the best possible growth of 

 the plant, but only in rare instances have the values of these optima 

 been detinitely fixed. It has been ccmsidered sufficient if we knew the 

 optimum temperature for the geiinination of our cultivated plants. 



WATER IN THE PLANT, 



The water of the soil })enetrates the jjliiHt, being drawn in through 

 osmosis and the aspiration resulting from the transpiration of the leaves. 

 Transi^iration assures an exit for the greater part of the water absorbed. 

 The amount of water in the interior of the plant therefore depends on 

 the relative intensity of absorption and transpiration. If the water 

 taken in by the plant dimhiishes, that given off' will also diminish, but 

 more slowly and in less amount. There is then ])roduced a kind of 

 dehydration, which, through the organic balance, tends to increase the 

 osmotic entrance of water for the rei^'stablishnient of the equilibrium 

 of concentration of the internal solutions. If there should happen to 

 be at the time a liick of water in the soil, the normal hydration of the 

 tissues is not sustained, and we say that the jdant suffers from drought. 

 On the other hand, ti-anspiration being excessive and absorption limited 

 the equilibrium of the sap current is disturbed, resulting in a tempo- 

 rary drying up, which becomes permanent if the cause persists for a 

 long time. The plant, on the other hand, suffers from excess of 

 moisture because there is a lack of ai^ration of the underground system. 

 Certain plants are specially adapted inorphologically to resist drought 

 and to maintain a constant proportion of internal moisture. This adap- 

 tation consists in the provision of morphological mechanisms, which are 

 designed to prevent too great variation in transpiration, as is admirably 

 shown in desert plants, which as a rule contain a normal amount of water 

 and are not dried out as they appear to be. The means employed by 

 the plant to control transpiration are well known but very variable. 

 The leaves are greatly reduced in size, or thickened as in fleshy plants, 

 in order to reduce the transpiring surface or to increase the reserve 

 water; the stems are gradually lignified, the stomata are placed in pro- 

 tective depressions, the greatly developed trichomes form a protective 

 screen, the epidermis has a thickened cuticle, the reserve water is stored 

 up in special reserve tissues or in the root, which is proportionately 

 developed. J. Vesque^ has discussed this special adaptation of the 

 plants to different water conditions, showing that plants change their 

 structure in order to adapt themselves to diminished water supply. 



•Anatinnio dcs tissus appliqiioo .'i l:v classificatiou ties plantcs, Nouv. Arcliiv. dn 

 Musenin, n. ser., 4 ami 5. L'espf-co ;iu point de vu de I'anatoinio compar<?e, Ann. sci. 

 nat. Bot., ser. 6, Vol. XIII. Numerous papers ou the role of water, Ann. sci. nat. Bot. 



4250— No. 1 2 



