TRANSPIRATION. 283 



MonotrojM, mentioned earlier on, which stands in such a peculiar relation to the 

 mycelium of fungi. On the other hand, in most green flowering plants which bear 

 leaves, a complete replacement of transpiration, continuing for a long time, is not an 

 advantage. Experience has shown that green leafy plants, when kept for a long 

 while in an atmosphere saturated with vapour, cease to grow and become unhealthy; 

 they lose their leaves, and at length succumb altogether. This happens even if the 

 amount of light, the temperature of the atmosphere and of the earth, the composi- 

 tion and humidity of the soil, in short, if all the other conditions of life are the 

 most favourable that can be imagined for the plants in question. It follows from 

 this that it is not immaterial to leafy plants how the sap reaches the leaves, 

 whether it is drawn up by transpiration, or forced up by root-pressure. If the leaf 

 transpires, water, in the form of vapour only, is given off to the atmosphere ; all the 

 materials which have been brought in solution from below to the leaves remain 

 behind in the cells of the leaf. If, on the other hand, fluid water is pressed from 

 the pores of the leaves by root-pressure, salts, sugar, and other compounds are always 

 to be found in the exuded drops, having passed through the cell-wall in solution in 

 the water. When it is a question of secreting sugar as a means of alluring insects, 

 or salts for a protective covering, such an exudation cannot advantageously be given 

 up, but is on the contrary a fundamental part of the economy of the whole plant. 

 If this is not the case, and if materials which have a part to perform in the leaf by 

 the formation of organic substances are exuded with the drops of water, and the 

 drops falling from the surface of the plant trickle to the ground, there is loss of 

 material, which does not contribute to the advantage, but rather to the detriment, 

 of the organism. 



The signification of transpiration may be explained in this way. By transpira- 

 tion not only is water brought from below to the more highly situated parts of the 

 plant, but nutritive salts in solution are also conducted to the green tissue of those 

 branches and leaves which are exposed to light and air. The greater part of the 

 ascended water is only used as a medium for the transmission of mineral salts, 

 which have been taken from the soil into the plant. When it has reached the 

 leaves, most of the water evaporates into the atmosphere, while the salts conducted 

 by it into the green tissue remain behind, in order to take part in the chemical 

 changes by which organic compounds are manufactured out of the raw materials. 

 These salts are indispensable here, and transpiration is therefore also necessary in a 

 corresponding degree. Without transpiration, it would be impossible that plants, 

 whose green branches and leaves are surrounded by air, or that trees, which rank 

 before all other plants on account of their superior size, could be properly nourished; 

 consequently transpiration must be regarded as one of the most important life- 

 processes of terrestrial plants. 



