ABSORPTION AND MOVEMENT OF WATER 139 



plants are so equipped. Whenever the temperature of the 

 plant is higher than that of the moisture-saturated air 

 outside, transpiration will take place, the water-vapor 

 condensing on the surfaces of the plant and elsewhere. 

 Because the plant in its respiration has a means of develop- 

 ing heat, it must often be the case in the tropics that the 

 plant is warmer than the surrounding air. That transpira- 

 tion into the moisture-laden air of the tropics is sufficient 

 for getting rid of water is evident when we take into con- 

 sideration the following matters. First, the plant absorbs 

 water because it needs and uses both water and the salts dis- 

 solved in it. Of the salts it needs only very small amounts, 

 as shown by culture experiments, and though analyses 

 of the mature plant may reveal the presence of much larger 

 amounts of some or all of the useful salts, it does not by 

 any means follow that these amounts are used, or, if they 

 are used, that the plant is not over-fed. Of the water it 

 needs enough to bring adequate amounts of the indispens- 

 able salts, and if the plant absorb enough water for this pur- 

 pose, it will certainly have sufficient for aU other purposes 

 also, because the solutions of needed salts are so dilute. 

 In certain places one of the advantages attained by trans- 

 piration certainly consists in the lower body-temperature of 

 the plant, since vaporization is a cooling process. Second, 

 transpiration even into a very humid .atmosphere will suffi- 

 ciently concentrate the cell-sap of superficial cells to ensure 

 an osmotic current into these cells, supplying them with 

 both the water and the salts which they may need. Third, 

 no more water will be absorbed than the plant needs, for 

 the living cells control by their activities those physical 

 conditions which make absorption possible. If the trans- 

 piration of plants living in the humid tropics is less than of 

 plants living in drier regions, the absorption of water will 

 be less. So long as water and needed salts are absorbed 

 in sufficient quantities, growth and the other activities of 

 the plant will be normal. The luxuriant vegetation of the 

 tropics impresses every one with the idea that there growth, 

 food-manufacture, etc., must be more rapid and more abun- 

 dant than in temperate regions. The correctness of this 



