3SN 



CONDUCTING SYSTEM 



vascular system, between plants which grow in dry places and those which 

 affect damp localities. We are indebted to Zalenski for detailed observa- 

 tions upon this point. Zalenski expressed the development of vascular 

 tissue in terms of its linear dimensions per unit area of leaf-surface. 

 Some of his measurements are set forth in the accompanying table : 



Name. 



Chrysosplenium alter nifolivm, - 

 Maianthemum bifolium, - 

 Actaea spicata, - 

 Asplenium Filix femina, 

 Geranium palustre, 

 Trifolium repens, - 

 Psoralea bituminosa, 

 Reseda lutea, - 

 Astragalus austriacus, - 

 Scutellaria alpi?ia,- 



According to Schuster, a comparison between the vigorously trans- 

 piring "sun-leaves" and the feebly transpiring "shade-leaves" of 

 one and the same plant reveals similar differences as regards the linear 

 dimensions of the vascular system. 



The numerous anastomoses of the vascular network are of great 

 physiological importance. Apart from their mechanical significance, 

 which has already been discussed on a previous occasion (Ch. IV.), they 

 play an indispensable part in equalising the water-supply at different 

 points in the photosynthetic system, at any rate where the latter is 

 continuous over a large extent of surface. This last-mentioned function 

 of the anastomoses was tested by the author with the aid of the follow- 

 ing experiment. A number of leaves of Sycamore {Acer Pseud oplatanus) 

 were selected in the month of May, and either one or two of their 

 principal veins severed close to the base, care being taken that where 

 two veins were cut, these were not immediate neighbours. In the 

 absence of anastomoses, the regions of the leaf blades traversed by the 

 severed veins would inevitably have dried up in a short time, owing to 

 insufficient water-supply. As a matter of fact, the leaves seemed to 

 suffer no ill effects in consequence of the apparently serious injury 

 inflicted upon their vascular system. Whether shaded by other leaves 

 or exposed to daily insolation, they continued, so far as could be 

 judged, to functionate in a perfectly normal manner, and certainly 

 did not undergo the usual autumnal discoloration any sooner than 

 uninjured leaves on the same tree ; injured and uninjured leaves 

 also fell at about the same date in the autumn. When it is 

 remembered, how often individual conducting channels in foliage- 



