Miscellaneous. 537 



The sap, which on its way takes part in the nutrition of the first 

 organs developed, arrives in the leaves, in the green parenchyma of 

 which it is submitted to a fresh elaboration, or in the chlorophyll-cells 

 of the stem of the fleshy plants destitute of leaves. The carbonic acid 

 of the air is absorbed and then decomposed during the day ; its 

 carbon is retained by the saj) and its oxygen in great part rejected. 

 The sap, thus modified under the influence of respiration, takes its 

 course through the cortical cells, which it nourishes. It then aids 

 in the multiplication of the cells of the generative layer, which are 

 produced in horizontal series. A portion of these cells thus hori- 

 zontally multiplied forms a new layer of bark, the woody fibres and 

 medullary rays ; the others are converted into vessels in the following 

 manner. The excess of the descending sap which is not employed 

 in the nutrition of the newly formed cells, or in thickening those 

 first developed, descends through certain of the newly formed cells ; 

 it dilates them, perforates them, and makes them take all the 

 characters of vessels, so that these cells, which, during the first 

 phase of their development, resembled all the others, appear sub- 

 sequently to be of a totally diff'erent nature. 



It is this vascular formation, which takes place, as we see, from 

 above downwards, at the expense of cells originating from a multi- 

 plication in horizontal series, that has led the authors of the theory 

 of descending fibres to believe that these vessels, of which they did 

 not recognize the nature, were true roots of the buds or leaves. 



But all the sap absorbed by the old or new cells, whether for their 

 increase in size or thickness, or for the production of starch, albu- 

 minoid substances, &c., which are to serve for subsequent growth, 

 is not used up by the cells. These only assimilate a part of its 

 elements and reject the rest. It is this caput mortuum which, in 

 the form of resins, essential oils, &c., is collected in peculiar reser- 

 voirs, from which it is afterwards thrown outwards * ; or the un- 

 assimilated matters are taken up by the laticiferous vessels, which 

 carry them back into the vessels properly so called (this is the 

 venous circulation^. There these substances, which are usually 

 destitute of oxygen, are elaborated and oxidized by the action of 

 the oxygen derived from the air, which penetrates even to the 

 vessels by intercellular passages ; they become again fitted for 

 assimilation. It would be from their oxidation, as I have already 

 stated, that the carbonic acid rejected by plants during the night 

 would be produced ; that which is produced during the day being 

 decomposed on its passage into the leaves under the influence of 

 light ; its oxygen is poured out into the atmosphere together with 

 that arising from the decomposition of the carbonic acid taken 

 directly from the air by respiration. 



The vessels produced by the descending sap, serve in the following 

 years for the ascent of the juices. They are filled therewith as long 

 as the vegetation is very active, but usually empty themselves when 



* It is undoubtedly emissions of this nature and of this origin that con- 

 stitute what are called the excretions of the roots, of which agriculture 

 seeks to turn to account in the rotation of crops. 



