VESSELS OF CLIMBERS. TYLOSES 325 



wall is directly proportional to the radius, whereas the volume varies 

 as the square of the radius. As a matter of fact, it is well known 

 that the vessels in the stems of climbers are very wide in comparison 

 with those of ordinary plants, being indeed often readily visible with 

 the naked eye. The average width of the larger vessels as measured 

 by Westermaier and Ambronn of certain climbers is set forth in the 

 adjoining table ; for the sake of comparison, the corresponding figures 

 are appended for a few common deciduous trees: 



Average width 



of large vessels 



in fi. 



- 200-300 



158 



140 



85 



76 



60 



40 



28 



The vessels of young branches are in general narrower than those 

 of the older wood; the difference is probably connected with the fact 

 that the distance over which water has to be transported is less in the 

 former case. 



The ecologically conditioned variation in the width of the vessels 

 appears especially striking, when one compares transverse sections of 

 the stems of climbing and erect species belonging to the same genus. 

 Thus the total cross-sectional area of the larger vessels is six times as 

 great in Galium wparine as it is in G. verum. Similar differences may 

 even be observed in different organs of one and the same plant. It is 

 very instructive, for instance, to compare the width and number of the 

 vessels in the tendrils and in the leafy branches of Vitis vinifera. As 

 a glance at the adjoining figure (Fig. 131) will show, no better illus- 

 tration could be found of the manner in which different physiological 

 requirements, in respect of water-conduction, are reflected in the ana- 

 tomical structure of the organs concerned. 



' l 



5. Tyloses 



164 



The cavities of water-conducting elements often become blocked, 

 for varying distances, by more or less closely packed bladder-shaped 

 intrusions derived from the adjoining parenchymatous cells. These 

 intrusive vesicles, the development of which was first studied and 

 explained by Hermine von Reichenbach, are known as tyloses. Most 

 frequently they arise in connection with one-sided bordered pits, the 



