210 



The Living Plant 



energy-dissipation: presumably, therefore, transpiration is an 

 adaption to protection against injury from the over-plentiful 

 energy of sunlight. Each of these explanations has its merits and 

 its difficulties, and no one alone is sufficient. Probably the truth 

 will be found to involve some participation of all three; transpira- 

 tion may be fundamentally a process which the plant cannot 

 prevent, but that is no reason why the plant cannot employ 

 it, and even develop it highly, as an easy method of raising its 

 requisite minerals, and a convenient means for the dissipation of 

 superfluous energy. But this question, too, is one of the many 

 whose solution lies with the future. 



Transpiration, however, is not the sole method by which water 

 is removed from the plant. Everybody has noticed the clear 

 shining drops which bejewel the margins of Grape leaves on 

 mornings that follow hot days and cool nights; these drops are 

 commonly thought to be dew but are not. They show very 

 strikingly also on young plants of Nasturtium and seedlings of 

 Grasses, where they can be made to appear whenever desired, 

 simply by covering the actively-transpiring plants for a few 

 minutes by a cooled, darkened, or dampened bell-jar. In a great 

 many other plants, too, the drops appear and are mistaken for 

 dew. The slender wet streaks often seen on the leaves of the 

 Cannas just after sundown, come from similar marginal drops; 

 and a tropical plant is said to exist from which water is projected 

 in a very fine jet. In all of these cases the water is known to 

 come from inside the plant, and the process, known physiologic- 

 ally as guttation, is a result of the following conditions. On very 

 warm days the vigorous transpiration is accompanied by an 

 equally energetic absorption and transfer, but the comparatively 

 sudden check to transpiration caused by the cool of the evening 

 does not at once affect the absorption; therefore water continues 

 to be forced into the stems and leaves to an extent which might 

 prove a serious detriment were it not for an avenue of escape pro- 

 vided by openings existing in the ends of the veins, for it is here 



