Absorption of Water by the Plant Body

Direct Action of Light on Leaf Cells.

As already described light markedly increases transpiration by causing the stomata to open, but the question arises as to any possible direct effect on the mesophyll. It was claimed by Francis Darwin (r914) that in a leaf which had been vaselined on both sides, to block the sto mata, and then slit so that the mesophyll cells transpired directly into air, light still increased the rate of water loss by as much as 36% in the case of the leaf of ivy. Such a huge increase seemed very unlikely, so the matter was again investigated by Henderson (1926), who showed that, when all allowances were made for possible changes of temperature, light did produce an increased rate of evaporation from the mesophyll cells of the leaf to the extent of about 5%. The exact mechanism of this is obscure; it is probably some effect on the resistance of the protoplasm to the passage of water through it to the surface of the cell. In the light the cell wall would be more fully imbibed, i.e., wetter than in the dark.

Advantages and Disadvantages of Transpiration.—This question is often debated, the extreme positions being (I) that the process is an unavoidable evil, (2) that it is a physiological necessity. The idea of its physiological necessity is mainly based on the view that the water and the inorganic salts that the plant requires are taken in together in the form of the very weak solu tion which occurs in the soil, and that the excess water must be got rid of in transpiration. This view is based on the erroneous

assumption that water and salts enter the plant together. As a matter of fact the forces bringing about the entry of water and the entry of dissolved salts into the root cells are of an entirely different nature; the two must enter quite independently. Whether transpiration is an "evil" may be debated but it is perfectly clear that given the necessity of taking in gases from the air, the loss of water by plants is quite unavoidable. The same pores which allow the entry of gases must also allow of the passage out of water vapour. The plant could only stop such water loss by having a continuous cuticle, and then the supply of carbon dioxide for the manufacture of sugar and starch would almost be cut off. The loss of water by transpiration is a necessary re sult of the construction of the plant. It has been suggested that a rapid stream of water rising in the wood (see below) is necessary to convey the salts to the leaves, but it is found that a reduction of transpiration does not necessarily reduce the accumulation of salts in plants. It may be that some minimum rate of up ward water flow is necessary to transport the salts to the higher parts of the plant. Transpiration may also play some small part in keeping down the temperature of plants exposed to the direct rays of the sun.