TRANSMISSION OF THE FOOD-GASES. 367 



with this food only through these cord-like aerial roots which are sent down into 

 the soil. 



These few examples are enough to show that the construction of the stem and 

 roots stands most intimately related to transpiration, inasmuch as the transpiring 

 green tissue is effected by the structure. But since the construction of these plant 

 members, i.e. the architecture of the stem, is also dependent upon various other vital 

 processes to be described later, it would not be fitting to discuss their relations here 

 in detail, and their treatment must be postponed until a later section. 



5. CONDUCTION OF FOOD-GASES TO THE PLACES OF 



CONSUMPTION. 



Transmission of the food-gases in land and water plants and in lithophytes. — Significance of 

 aqueous tissue in the conduction of food-gases. 



It has been repeatedly pointed out that a division of labour occurs in all large 

 plants, so that one portion of the cells provides for the reception of water and food- 

 salts, another for that of food-gases, and yet another for the conduction and trans- 

 mission of fluid and gaseous nourishment to the places where they are consumed. 



How the aqueous food-salt solutions derived from the soil are brought to the 

 green tissue, what contrivances are thereby brought into action, and what 

 phenomena of plant-life are related to this conduction have been discussed, as far 

 as practicable, in the previous pages, and it now only remains to describe the 

 transmission of the gaseous food-materials. This is far more simple than the 

 conduction of the solutions of food-salts. The most important of the food-gases in 

 question are carbonic acid and nitric acid. Carbonic acid is continually being 

 conducted by means of water to the green tissues. The shortest passage is to be 

 found in aquatic plants whose protoplasm, provided with green chlorophyll and in 

 need of carbonic acid, is only separated from the surrounding water by a thin 

 cell-wall, while this water always contains carbonic acid, though perhaps only in 

 small quantity. Under the influence of sunlight, the groups of green cells in 

 hydrophytes form a centre of attraction to the carbonic acid, which is sucked up 

 with great energy from the surrounding water, passes easily through the cell- wall, 

 and so comes directly into the neighbourhood of the green protoplasm, i.e. that 

 place where its decomposition is effected. The green cells of water plants therefore 

 furnish an apparatus for both absorbing and decomposing carbonic acid, and 

 usually no further means and no special conduction through other cells are required. 



In lithophytes it is otherwise. Here we have the remarkable fact that they are 

 only active at times; only, that is to say, when they are sufficiently moistened by 

 rain, dew, and mist, and are to some extent submerged for a time by heavy down- 

 pours. In dry air their vital activity is suspended ; they then adhere to the rocks like 



