322 CONDUCTING SYSTEM 



may also be very long and wide in exceptional cases for instance, in 

 the stems and petioles of Musa, Canna and NelumMum (cf. above, 

 p. 308). 



The division of labour which finds expression in the differentiation 

 of vessels and tracheides, of course corresponds to a high level of general 

 organisation. In accordance with this rule, one finds that the water- 

 conducting tissue of Mosses consists entirely of rudimentary tracheides. 

 In the Pteridophyta, also, tracheides alone are recorded, with few 

 exceptions {Pteris aquilina, root of Athyrium filix femina). Among 

 Conifers true vessels are unrepresented, even in the primary bundles, 

 while the secondary woody cylinder likewise consists solely of " fibre 

 tracheides." It is, in fact, only among Dicotyledons that true vessels 

 predominate in the secondary xylem : in Salix, in Ficiis and in certain 

 Leguminosae they are the sole water-conducting elements. 



4. Vernations in the development of the water-conducting system in 

 relation to different requirements. 163 



The demands that are made upon the water-conducting capacity of 

 the vascular system, of course vary directly in accordance with the 

 transpiratory activity of the plant. Consequently, the size and number 

 of the foliage leaves, and the climatic and edaphic conditions to which 

 the plant is exposed, must exert a marked influence upon the quantita- 

 tive and qualitative development of the vessels and tracheides. Special 

 ecological relations, finally, and peculiarities in the general scheme of 

 construction of the plant-body, also affect the development of the paths 

 of water-conduction. 



The general correlation which exists between the number and width 

 of the water-conducting tubes on the one hand, and the extent of the 

 foliar transpiring surface on the other, stands in no need of explanation ; 

 great interest, however, attaches to the fact established experimentally 

 by Jost that this correlation sometimes affects the differentiation of 

 the vascular system during the development of the individual in a very 

 remarkable manner. If a seedling of Phaseolus multiflorus is deprived 

 of one of its two primary leaves, or even of later-formed leaves 

 together with one or more axillary buds, in either case before the 

 organs in question have unfolded it is found that the vascular bundles 

 supplying the amputated leaves or buds remain rudimentary, especially 

 as regards their water-conducting components. Jost obtained confirma- 

 tory results with seedlings of Heliantlius annuus and Vicia Faba. In 

 these cases there is evidently a process of adaptive self-regulation at 

 work, the elimination of the transpiring surfaces leading to a timely 

 arrest of the development of the corresponding water-conducting 

 strands. 



