42 METABOLISM 



light operates still more in an indirect way, i. e. as a stimulus to the guard- 

 cells. In many plants closing of the stomata has been observed to take place 

 on darkening. Darkening must, therefore, also be recognized as a stimulus, 

 since after osmotically active bodies have been manufactured in light, they 

 cannot be removed so quickly, after exclusion of light, as to account for the 

 rapidity with which the stomata again close. According to Leitgeb (1886) 

 the closing of the stomata in the dark is passive, owing to the guard-cells being 

 pressed together by the turgor in the neighbouring cells. An increase in tur- 

 gidity in darkness is a very general and easily explicable phenomenon. 



In addition to the effect of light and atmospheric moisture on the width 

 of the stomatal aperture that of changes of temperature also has received 

 attention, but into that and related subjects we need not enter, those already 

 discussed being the most important. In spite of the voluminous literature 

 on stomata, detailed researches on these organs in representatives of the chief 

 biological groups of plants are still wanting. We know enough, however, to 

 be able to affirm without fear of contradiction that in stomata plants possess 

 a remarkably valuable apparatus for regulating transpiration, provided the 

 external conditions for absorption and transpiration be approximately normal, 

 that is to say, such as do not approach the extremes to which the plant 

 cannot adjust itself. Any attempt to cultivate some of our economic plants, 

 such as cereals or tobacco, under the conditions prevalent in a desert, or in 

 the saturated atmosphere of a tropical forest would, undoubtedly prove 

 a failure. At the same time, a study of plants which are naturally fitted to 

 live under such extreme conditions discloses to us a variety of adaptations 

 calculated, on the one hand, to limit transpiration as much as possible, and, on 

 the other, to further it to the utmost. A brief sketch of these adaptations may 

 be appropriately introduced here ; the reader is referred for details to the 

 works of Haberlandt (1896), Schimper (1898), and Stahl (1893, 1896). 



Reduction in the amount of transpiration may be effected by a reduction 

 in the number of stomata or by an alteration in their anatomical structure. 

 Further, plants which live in dry regions usually have well-developed cuticles, 

 whose power of retarding transpiration is increased by the deposition in, or on, 

 them of wax ; moreover, the capacity for imbibing water may be reduced in such 

 parts of the plant as are exposed directly to air. Development of hairs full of 

 air can also effectively retard transpiration, since such a covering protects 

 the plant from the effects of air currents, producing a superficial region free from 

 atmospheric movement. In addition to such preventatives dependent on cell 

 structure we find also adaptations dependent on the form and arrangement 

 of the parts concerned. A fiat extension of the foliage leaf greatly favours 

 transpiration, more especially if the whole leaf surface be exposed to the 

 sun's rays ; but plants are known to exist which avoid strong insolation by 

 presenting the edges of their leaves to the sun (compass plants. Eucalyptus, 

 &c.. Lecture XXXVI), and especially, as need arises, by changing the position 

 of their leaves with relation to the direction of the incident ray (Lecture XXXVI). 

 More effective still is the power which certain plants possess of reducing their 

 surface by rolling up leaves which are normally extended, or the extended 

 form may be avoided altogether and a spherical shape assumed (Cactaceae, 

 Euphorbiaceae, &c.). 



On the other hand, in plants which inhabit very damp situations we find 

 adaptations for furthering transpiration ; as, for example, the special form and 

 arrangement of leaves for aiding in the rapid drainage of water from their 

 apices ; coloured cell-sap and consequent increased temperature of the cells 

 concerned ; thin, easily permeable cuticle ; increase of the epidermal surface ; 

 complete exposure of the stomatal guard-cells, &c. 



It is of the utmost importance that the plant should be able, according to 



