CHAP. IL] VEGETAL NUTRITION. 101 



that which is produced after the death of the plant, and which 

 has for result the mineralisation more and more complete of the 

 organised tissues, deprived of life. In the last case special 

 chemical combinations are produced : first of all, a ternary 

 compound, ulmine (C 40 H 16 O 14 ), which is transformed into a 

 series of derived products more and more oxygenised (ulmic 

 acid, humine, humic acids, and so on). Such of these products 

 as are acid join themselves to the ammonia formed also during 

 the cadaveric decomposition of the plant ; they then constitute 

 soluble salts, which can be reabsorbed by the roots, and thus be 

 restored to the vital movement. 



To terminate this abridged description of vegetal respiration, 

 it remains for us to indicate by what paths the air introduces 

 itself into the mechanism of the vegetal tissues. 



In the inferior plants, the mosses and the conf ervse, there is no 

 aerian circulation, no special apparatus. The oxygen is absorbed 

 direct by the cells. In the complex plants, having true roots, 

 true aerian leaves, canals, there is a commencement of functional 

 specialisation. The air is absorbed a little by the bark, but 

 especially by the leaves. Th. de Saussure, when analysing the 

 interstitial air of plants, found that the air the least modified, 

 the least poor in oxygen, is that of the leaves ; that of the stems 

 is less oxygenised, and that of the roots still less. 



The air probably penetrates into the phanerogamous plant by 

 the stomata of the inferior surface of the leaves. Thus if we 

 plunge leaves into water and if we subject them to the action of 

 the pneumatic pump, we see the air escaping regularly in bubbles 

 by the section of the vessels of the petiole (Dutrochet). Once intro- 

 duced by the stomata, the air circulates in the lacunae, the inter- 

 cellular meatus, especially in the tracheae and the punctuated 

 tubes, when the first flow of sap in spring has forsaken them. 



In the aquatic leaves destitute of epidermis and of stomata 

 the aerian water acts direct on the cells, with thin walls, of the 

 parenchyma. When there are stomata the process is the same 

 as in the open air. 



