24^ LECTURE XV. 



As the moist walls of the assimilating cells transpire their water into these 

 spaces, they are enabled to attract a fresh supply of water from the wood 

 bundles, the nutritive materials of which are absorbed in the interior of the as- 

 similating cells. It is easy to see that in this way — that is, by the formation of 

 vapour in the intercellular spaces — a more uniform supply of food to each cell is 

 rendered possible, than if the water evaporated at the surfaces of the leaves them- 

 selves. Since, now, this aqueous vapour, arising at the proper places, can only escape 

 through the stomata, the possibility is afforded of concentrating the supply of 

 nutritive matters brought by means of the transpiration at those times when, under the 

 influence of intense light, assimilation is actually taking place in the cells containing 

 chlorophyll : for at this time the stomata are open, while they become narrowed 

 in the shade, and closed in darkness. The mechanism of the opening and closing 

 of the stomata is thus adapted to ensure a flow of nutritive materials from the 

 soil to the assimilating cells, at those times when, through the open stomata, the 

 entrance and exit respectively of the carbon dioxide of the atmosphere and the 

 oxygen set free by its decomposition are much facilitated. 



It would be extremely rash to suppose that, because submerged water-plants 

 possess no stomata and do not transpire, the considerations here put forward are 

 unimportant. The mode of life and organisation of water-plants are both essentially 

 different from those of the land-plants. The transpiration and corresponding flow 

 of material which they lack, are simply replaced by their being able, by means 

 of their feebly cuticularised epidermis, to take up water and substances dissolved in 

 it from outside at all parts; accordingly all those arrangements are absent which 

 we are here studying simply as conditions of the life of land-plants. The im- 

 portance of our considerations is not altered by the fact that small quantides of 

 aqueous vapour can escape from the epidermis even without the aid of stomata. 

 The question as to what extent this happens and whether it is of any use at all for 

 the plant may, in fact, be altogether neglected. It may indeed be said that in no 

 region of vegetable physiology has the concern for insignificant minutiae led people 

 so far astray from the insight into the great significance of the really important 

 matters of organisation, as in connection with this matter of transpiration and all 

 the concomitant phenomena. All possible minutiae have been studied ; but the 

 principal fact, that it is a matter of the supply of nutritive matters to the organs of 

 assimilation has been scarcely noticed. Since, however, as is clear from the pre- 

 ceding considerations, it is just the opening and closing of the stomata which 

 regulate the transpiration, and with it the ascending current of water, and finally 

 the absorption of nutritive matters from the soil, we will briefly enter a little more 

 in detail upon the mechanics of these movements. 



The opening and closing of the stomata are brought about by alterations in the 

 form of the guard-cells, the nature of which is to be perceived most clearly from the 

 figure here given ^ (Fig. 203). The thick lines mark the contours of the guard-cells in 

 transverse section, at right angles to the surface of the leaf when the aperture is open. 



* The statements in onr text on the structuie of the stomata, so far as the mechanics are con- 

 cerned, are taken freely from Schwendener's treatise, ' Ueber Ban und Mcclianik der Spaltliff- 

 >!i(figen,' Monatsbericht d, Kgl. Akad. der Wiss. zu Berlin, July, 18S1. 



