370 TRANSMISSION OF THE FOOD-GASES. 



the upper side of the leaf. But it must not be forgotten that the same process also 

 takes place on the under side of the leaf, particularly when the green tissue is not 

 divided into palisade cells and spongy parenchyma, and also when the epidermis is 

 provided with stomata both on the upper and under sides of the leaf. In certainly 

 70 per cent of all leafy plants the arrangement is such that palisade tissue occurs 

 beneath the succulent epidermis of the upper side, under this again spongy 

 parenchyma, and again under this the epidermis of the lower side, which is 

 abundantly pierced by stomata. It can therefore be asserted, for the majority of 

 plants with green foliage, that the epidermis of the upper side chiefly regulates the 

 transmission of carbonic acid to the palisade cells, and that transpiration is chiefly 

 regulated by the epidermis of the lower side. 



It is hardly probable that carbonic acid finds entrance to the green tissue 

 through the stomata. At the time when the demand for carbonic acid is at a 

 maximum in the green tissue, a considerable quantity of food-salts must be 

 delivered to the green cells, and the water which provides for the transport of the 

 food-salts from the soil up to the small chemical laboratories, as the palisade cells 

 may be called, is rapidly expelled from the stomata in the form of vapour. But 

 while water-vapour is streaming out of the stomata, the carbon dioxide of the air 

 can hardly stream in through the same avenues at the same time, and it may be 

 concluded that when, generally speaking, this gas is absorbed through the stomata, 

 the occurrence is exceptional. 



Concerning the filling of the epidermal cells with water and carbonic acid, it 

 should be here again pointed out that in not a few plants the absorption of rain and 

 dew takes place directly through the foliage-leaves. Since rain and dew always 

 contain small quantities of carbonic acid and traces of nitric acid, this method of 

 filling the epidermal cells is so much the less to be undervalued. In very many 

 green foliage-leaves the continuous epidermis above the palisade cells is capable of 

 being moistened, while the lower epidermis, rich in stomata, on the other hand, is 

 kept dry by the most varied contrivances; and it is very probable that in such cases 

 the water of rain and dew is taken up by the whole epidermis of the upper leaf- 

 surface, especially when these epidermal cells have a short time previously given up 

 a portion of their contents to the green tissue, and have become consequently 

 somewhat collapsed. In many cases it must be concluded, from their shape and 

 position, that the filling of the epidermal cells is only caused by the watery sap 

 brought from the roots, and indeed only by means of the green palisade tissue, 

 i.e. of the same tissue which, on occasion, again receives watery fluid from the 

 epidermal cells. This periodic alternation of absorption and expulsion may be 

 explained in the following manner. The water arriving from the soil is given off 

 by the palisade tissue to the epidermal cells at certain times, i.e. when no carbonic 

 acid is required, in order that carbon dioxide may there be drawn from the air and 

 changed into carbonic acid. When this has happened, and a demand for carbonic 

 acid is set up in the palisade tissue, this tissue takes back the water it had 

 previously given off, now of course accompanied by the absorbed carbonic acid. 



