XI. THE PLANT 227 



these small apertures, diffusion of dissolved matter in the protoplasm 

 itself will take place at a rate probably far greater than is proportional 

 to their area. The apertures, in fact, as suggested by Brown and Es- 

 combe, 1 play a similar part to the stomata in the leaves in promoting 

 diffusion without interfering with the structural advantages of the cell 

 walls. 2 



2. The Stem, from our present aspect, may be regarded as the 

 mere means of communication between the roots and the leaves. It, 

 however, serves many other purposes, e.g., often as a receptacle for 

 reserve material or for excreted matters, and to some extent, when 

 green, as an assimilative agent. 



3. The Leaves are the seat of the most important chemical 

 changes occurring in the plant. It is here that the reactions charac- 

 teristic of vegetable life mainly take place the formation of carbo- 

 hydrates from carbon dioxide and water, of amides and albuminoids 

 from the same constituents and the nitrates and sulphates taken in 

 through the roots. They also fulfil another most important function 

 as a means by which the water absorbed by the root may be got rid 

 of by evaporation. This process of transpiration, as it is called, takes 

 place at a rate which depends upon several circumstances, among 

 others, upon the hygrometric state of the atmosphere round the leaf. 

 The absorption of carbon dioxide and the evaporation of water take 

 place mainly through the minute openings on the exterior of the leaf 

 known as stomata. Each stoma is provided with two guard cells, by 

 the varying turgescence of which the size of the opening can be regu- 

 lated. If the uard cells become flaccid, as they tend to do by exces- 

 sive loss of water by transpiration and also in the dark, they more 

 nearly close the opening the stoma between them and so lessen the 

 passage of water vapour outwards and of carbon dioxide inwards. 

 The space below the epidermis of a leaf is surrounded by cells which 

 contain chlorophyll, and it is the energy of light absorbed by this 

 colouring substance which effects the decomposition of carbon dioxide. 

 It is found that the assimilation of carbon dioxide is most active in 

 just those rays of the spectrum of white light which are absorbed by a 

 solution of chlorophyll. This is well shown in the accompanying 

 diagram, Fig. 6, which gives the relative assimilation by plants in dif- 

 ferent parts of the spectrum (continuous line), and also the relative 

 absorption by a solution of chlorophyll (dotted line). It will be seen 

 that the two curves correspond very closely. 3 



The next diagram 4 (Fig. 7) gives the distribution of the inten- 

 sities of the various rays of the solar spectrum as regards assimilation 



of carbon (continuous line ), brightness as measured by the eye 



(broken line ), heating effects (dotted line ), and 



chemical action on silver salts (broken and dotted line . . . ). 



These two diagrams do not agree as to the position of the maxi- 



1 Phil. Trans., 193 (1900), 280. 2 Vide p. 229. 



3 Engelmann, Bot. Zeitung, 1884, 80. 4 Sachs, Plant Physiology, p. 305. 



