138 PLANT PHYSIOLOGY 



outer surface of which is rendered as impermeable as possi- 

 ble to water and water-vapor by the waxy covering of the 

 outer walls of the epidermal cells, by greatly thickening and 

 cutinizing these walls, by forming more than one layer of 

 epidermis, by sinking the guard-cells of the stomata to the 

 second layer of epidermis, by greatly reducing the number 

 and size of stomata, and in some cases by forming a woolly 

 covering of dead hairs which still further insulates the tissues 

 within. From such plants the loss of water is slight in pro- 

 portion to the mass, less in proportion to the surface, least 

 in proportion to the amount of water contained in .the 

 plants. Evaporation from these plants is reduced to the 

 minimum, transpiration is lowest in rate and in volume, 

 first, by reason of the composition of the plant-body, slimy 

 and gelatinous materials holding water, and water-storing 

 tissues, forming a considerable part of the volume of the 

 plant ; second, by the area, composition and covering of the 

 surface ; third, by the small number of openings through the 

 insulating covering ; and fourth, by the body-temperature of 

 the plant being lower than that of the air when the air 

 could otherwise take up most moisture.* 



The opposite extreme is represented by plants living in the 

 very humid regions of the tropics. There the air is always 

 near the point of saturation, and the almost daily showers 

 at certain seasons attest both the frequency with which the 

 air attains the point of saturation and also the great 

 amount of moisture which can be quickly precipitated. The 

 well-known copiousness and frequency of the tropical down- 

 pours indicate that great volumes of water must somehow 

 be vaporized. It has sometimes been concluded, from the 

 tardy evaporation of water from wet masses having the 

 same temperature as the surrounding air, that all tropical 

 plants must have other means than transpiration for get- 

 ting rid of the water absorbed by them. Some do have 

 other means in the hydathodes (p. 128), but not many 



* For a further discussion of these interesting adaptations consult 

 Goebel, Pflanzenbiologische Schilderungen ; Volkens. Flora der agyptisch- 

 arabischen Wuste, 1887; Schimper. Pflanzengeographie auf physiolo- 

 gischer (jrundlage, 1898. 



