202 The Living Plant 



most attractive kinds there are, but are the most attractive that 

 can withstand the dryness that prevails in our houses in winter, — • 

 a dryness that is due not so much to the heat of the house as to 

 the fact that the general atmosphere in the winter has a very low 

 content of water vapor. A house plant in fact is one whose 

 transpiration in that dry heat is no greater than can be com- 

 pletely compensated by the absorption and conduction of water 

 from the soil. And this relation of transpiration to conduction 

 explains another notable phenomenon in plant nature, namely 

 the limitation in the height of trees, which in general are just so 

 high as the water can be conducted in sufficient abundance to 

 supply the transpiration from the foliage. When that height is 

 reached the tree can still spread out laterally, which explains the 

 flat tops of the largest Elms, IMaples, Oaks and others, and of 

 many forest trees when seen from mountain tops. A transpira- 

 tion effect of a very different sort is displayed by a good many 

 plants in the early spring. It is a fact that roots absorb water 

 very slowly when chilled, and if they are kept for a time at a low 

 temperature, while leaves and stems are exposed to conditions 

 favorable for transpiration, as is effected quite easily by experi- 

 ment, the plants will wilt very rapidly. These very conditions 

 are often supplied naturally in the spring, for if the soil remains 

 frozen or very cold after warm bright days have forced out the 

 leaves, or if a cold spell that chills the soil is followed abruptly 

 by very warm bright windy days, then the young leaves transpire 

 so much faster than the water can be supplied by the roots, that 

 they become dry-blasted as if by a frost, to which latter cause, 

 indeed, this effect is commonly but mistakenly ascribed. This is 

 the explanation also of the fatal browning of the leaves of many 

 ornamental evergreens, whose leaves are awakened to active 

 transpiration before the roots can supply the water they need; 

 and it is, indeed, a chief cause of winter-killing generally. And 

 finally, as to transpiration effects, there is one more way in 

 which this process exerts a very remarkable influence upon 



