38 METABOLISM 



stomata, for if these be blocked up by vaseline or tallow the stream of 

 bubbles at once ceases. It must also be noted that other exits from the 

 aeriferous system, in addition to stomata, are known to occur in plants. 

 (' Pneumachodes' ; compare Haberlandt, 1896). 



At the present moment the only gaseous exchange taking place through 

 the stomata which we need consider is the giving off of water vapour. It 

 follows from the structure of the plant as described above that, in addition to 

 the transpiration taking place from epidermal cells, there must also be a certain 

 amount of ' internal transpiration ', since each cell wherever it borders on an 

 intercellular space will give off water vapour, the immediate result being the 

 saturation of the air in the intercellular spaces. The whole plant thus loses water 

 whenever water vapour escapes through the stomata from the intercellular 

 spaces. Obviously only a minute quantity of water can pass to the exterior 

 through a single stoma since the slit is so small. The diameter of the largest 

 of them (e. g. those of Amaryllis) is only o-oi-o-02 mm., and the openings are 

 so minute that a needle prick appears as a huge hole in comparison. The 

 significance of the stomata in the vital economy of the plant, apart from charac- 

 teristics which will be studied later, depends on their immense number. In 

 the situations in which they are most abundant, e. g. the underside of the 

 foliage leaf, we find on an average from 40 to 300 per sq. mm., but in extreme 

 cases there may be as many as 625 {Olea) or 716 {Brassica rapa) in the same 

 area. According to Noll (1902) an average-sized leaf of Brassica rapa has 

 no fewer than eleven millions of stomata, while a leaf of the sunflower has 

 thirteen millions. It must be remembered in this connexion that Brown and 

 EscoMBE (1900) have shown that diffusion of gases through a plate pierced 

 by numerous fine pores takes place as rapidly as if the spaces between the 

 pores were non-existent. 



We must also differentiate between an epidermal and an intercellular, or, in 

 other words, between a cuticular and stomatal transpiration, and the difference 

 may often be recognized by contrasting the behaviour of opposite sides of the 

 leaf. Many foliage leaves bear stomata only on their undersides, and if the 

 cuticle be alike on both sides, we may assume that we have a cuticular tran- 

 spiration taking place from the upper side, and a stomatal transpiration 

 taking place from the lower as well. Several methods of investigation, most 

 conveniently perhaps the cobalt-chloride method, demonstrate, however, 

 that cuticular transpiration is frequently so small that it may be taken as 

 practically nil. If a piece of blue cobalt-chloride paper be placed on the under- 

 side of a leaf of Liriodendron tulipifera it becomes red in a few seconds, while 

 a piece placed on the upper side remains blue for several hours, the general 

 conditions being the same in each case. Plants which live in damp air, e. g. 

 the Hymenophyllaceae, possess much thinner cuticles than those which live 

 in drier air, hence in their case cuticular transpiration is very obvious and 

 readily capable of demonstration by cobalt paper. The extreme case is shown 

 by submerged plants and by roots where the permeability of the cuticle (in the 

 absence of stomata) to water is shown at once by the rapidity with which 

 wilting takes place. 



Even assuming that the nature of the cuticle, and the number and dimen- 

 sions of the stomata in any plant are known, the absolute amount of transpira- 

 tion cannot be determined without knowing as well what the external conditions 

 are, since the amount of transpiration varies extremely with alterations in these. 

 The way in which many of these external factors affect transpiration is obvious, 

 for they may be observed just as readily in purely physical experiments with 

 a substance capable of absorbing water, e.g. glue or filter paper ; in the plant, on 

 the other hand, the influence of external factors introduces remarkable compli- 

 cations, due to the fact that the organization of the plant is, in the first instance. 



