66 



PLANT PHYSIOLOGY AND ECOLOGY 



known amount of water, and the other moist soil of known weight. 

 Of the remaining plants, place one in air as dry as possible, one in a cool 

 spot, one in a windy situation, and one in darkness. Weigh each at 

 the same intervals. Determine the day and night rate of loss, and 

 the total loss for each plant, for the water surface, and for the soil surface, 

 basing them upon a square decimeter of surface. Explain the results 



Fig. 18. Portable box and balance for measuring transpiration in 



the field. 



in the different situations. Compute the ratio of loss from one square 

 decimeter of the check, from the water, and from the soil surface. 



83. The amount of transpiration in plants. The amount of 

 water transpired differs for the individual as well as for the species. 

 Much of this variation arises from differences between habitats, 

 but species of the same habitat differ widely, entirely apart from 

 any variation in physical factors. This is clue to the fact that 

 species differ both in the amount and character of their tran- 

 spiring and absorbing surface. The same is true, though to a 

 much smaller extent, of plants belonging to the same species. 

 Aside from distinguishable differences of structure or surfaces, 

 plants of the same species lose water in varying amounts, some- 



