REDUCTION OF THE WATER-CONDUCTING SYSTEM 323 



The water-conducting system likewise undergoes reduction if a 

 diminution or complete stoppage of transpiration is brought about, not 

 by the removal of the transpiring organs, but by an increase in the 

 humidity of the atmosphere, or by actual immersion in water. Kohl 

 has demonstrated this point by comparing the structure of the stem in 

 plants of Lamium album, Isopyrum tludictroides, Aster chinensis and 

 Lyeopus europacus, grown in a damp and in a dry atmosphere respec- 

 tively. A very considerable adaptability in this respect has also been 

 observed, by Schenck, in the case of Cardamine pratensis ; if individuals 

 of this ordinarily terrestrial plant are grown in water, the water- 

 conducting tissue of the stem becomes greatly reduced. As might be 

 expected, plants which are naturally amphibious exhibit a similar 

 plasticity. 



The most far-reaching reduction of the water-conducting strands is 

 naturally to be found among submerged water-plants. In the majority 

 of such plants, spiral and annular vessels are indeed differentiated in 

 the young axial organs, but soon degenerate completely over long dis- 

 tances, being replaced by a lysigenous intercellular canal. This con- 

 dition is exemplified by certain species of Potamogeton, and by the 

 genera Zannichellia, Althenia and Cymodocea ; here the vessels only 

 remain intact at the nodes. In Elodca canadensis one or two axile 

 vessels are differentiated in the apical region, but disappear altogether, 

 even at the nodes, when the stem begins to elongate. According to 

 Sanio, CeratophyU torn never forms any vessels at all. It should be 

 noted that this vascular reduction by no means involves the tissues that 

 serve for the conduction of proteins ; as a matter of fact, the translo- 

 cation of these substances is not in the least affected by the nature of 

 the medium in which the plant grows. 



The fact that vessels are still differentiated in the young internodes 

 of most submerged Angiosperms, might be regarded as the consequence 

 of a hereditary tendency, since these plants are like all aquatic 

 Phanerogams undoubtedly descended from terrestrial ancestors. It 

 is, however, unlikely that the vascular tissue of the young stem is 

 altogether functionless, even in such cases. The frequent occurrence 

 of so-called water-pores or similar apertures in connection with the 

 foliar bundle-ends, furnishes indirect evidence of an actual flow of 

 sap along these transitory strands. The openings in question evidently 

 serve for the exudation of sap, a process which may to some extent 

 assist in the transportation of food materials, while it also facilitates the 

 removal of metabolic by-products. 



The development of the water- conducting system is noticeably 

 affected, not only by a diminution, but also by any increase in the 

 demands which are made upon it. Instructive illustrations of this 



