CONDITIONS OF TRANSPIRATION. 227 



that they facilitate and regulate the formation of vapour, in order to render 

 possible the inflow of fresh nutritive water to the organs of assimilation according 

 to requirement. Where this is not necessary, as in submerged water-plants, the 

 arrangements mentioned are also wanting ; and in land-plants which possess a large 

 transpiring surface, and are therefore devoid of a large assimilating surface, as 

 in species of Cadus and Crassulaceae, nutrition and accordingly growth also 

 are relatively feeble. When it has been sought, very thoughtlessly, to lay stress 

 upon the fact that land-plants assimilate and grow even in an atmosphere 

 saturated with vapour, where transpiration from the assimilating surfaces is not 

 possible, it has been overlooked that in such cases it is a matter of an extremely 

 feeble assimilation only, and accordingly also of a very small supply of nutritive 

 water to the leaves, — a supply so small that, as can be demonstrated, it is main- 

 tained without transpiration-currents. Moreover, there is the fundamental error 

 of assuming it as at all possible to maintain a land-plant even but 50 cm. high 

 in a space constantly saturated with aqueous vapour : the fact being that even in this 

 case (since saturation with vapour is effected with extreme difiSculty) opportunity 

 occurs for transpiring and maintaining a feeble flow of water. Let it but be attempted 

 to bring any tree, or even only a Sunflower or a plant ol Ricinus, to normal vigorous 

 development with actual prevention of all transpiration, and it will then be seen what 

 comes of it : every horticulturist knows that land-plants grown in very moist air, and 

 therefore with feeble ascending currents of water, are much too poor in substance 

 and too watery to pass as healthy plants. 



Considering the great importance of the movement of water called forth by 

 transpiration, for the whole well-being of land plants, the chief relations of organi- 

 sation of which are subjected to this function, it is worth while to consider the 

 phenomena in question more closely, and above all to examine those organic 

 arrangements which specially serve to produce the movements of water, as well as 

 to become acquainted with the mechanics of the remarkable movement itself. 



In the first place, it is necessary to convince ourselves that the leaves con- 

 tinually exhale considerable quantities of aqueous vapour. It is only necessary to lay 

 a leaf on the scale of a balance, to notice that it becomes continually lighter, 

 and finally dries up entirely ; this simply takes place by the escape of aqueous vapour. 

 In the same way a plant eventually withers and dries up when rooted in a flower- 

 pot and not watered. If a plant is allowed to grow in a glass or metal vessel 

 filled with nutritious soil, so that a large number of green leaves become developed, 

 and the upper surface of the vessel is then closed by a lid consisting of two halves, 

 which only allows the stem to pass through the centre; and if the whole is then put on a 

 scale, first placed in equilibrium, it is noticed that the scale-pan loaded with the plant 

 becomes lighter, and rises, although no water can evaporate from the vessel enclosing 

 the roots. Evaporation takes place, however, through the leaves ; and if a Tobacco 

 plant, for instance, has been used for this investigation, the total leaf-surface of which 

 amounts even to only a few square millimetres, and the experiment is arranged in 

 ordinary daylight, or in sunshine, the balance shows a loss by transpiration of one or 

 several cubic centimetres of water per hour, and it is easy to calculate that in the 

 course of a few days the same plant evaporates from its leaves several hundred cubic 

 centimetres of water, amounting to more than its own volume. This result is obtained 



Q 2 



