IMPORTANCE OF ANASTOMOSES 



389 



leaves must be rendered inactive, under natural conditions, by the 

 attacks of insects or by hail, it will be readily admitted that the 

 anastomoses are of the greatest importance, not only for the welfare 

 of the individual leaf, but also as regards the economy of the 

 whole plant. It is true that Vochting and Simon have recently 

 shown, that when vascular bundles are severed in certain stems and 

 roots, the continuity of the interrupted water-conducting channels may 

 be re-established by the formation of new 

 tracheidal connecting strands. Freundlich 

 has observed a similar reconstruction of 

 interrupted water-channels in some 

 Dicotyledonous leaves, whereas this re- 

 generation does not appear to take place 

 in the cases of Monocotyledons or Pteri- 

 dophytes. 180 Such observations do not, 

 however, in any way detract from the 

 importance which has been assigned to 

 the anastomoses. The formation of fresh 

 tracheidal connections involves consider- 

 able delay, during which any parts of the 

 leaf that had been cut off from their 

 direct water-supply would be exposed to 

 the danger of drying up, if no vascular 

 anastomoses were present. In any case, 

 the injuries to which leaves are subject 

 under natural conditions, are often so extensive that they could not 

 possibly be repaired by the formation of new vascular connections. 



In flattened, photosynthetically active axial organs, such as the 

 cladodes of Biiscus, the arrangement of the vascular bundles is exactly the 

 same as in fiat foliage-leaves, a fact which requires no explanation from 

 a physiological point of view. 



Fig. lot'. 



A. 



Course of the vascular bundles in a 

 petal of Barbarea vulgaris. B. Small 

 portion of a leaf of the same, drawn to 

 the same scale, for comparison with A. 

 xlO. 



V. THE CONDUCTING SYSTEM IN THE THALLOPHYTA. 1 ^ 



In many Khodophyceae and Phaeophyceae the central tissue of 

 the thallus-branches contains certain rows of elongated cells, with trans- 

 verse septa which display a considerable resemblance to sieve-plates. 

 These cell-series are evidently the representatives of the conducting 

 system of the Higher Plants, or rather of one of the most remarkable 

 constituents of that system, namely the sieve-tubes. 



Among the PtHODOPHYCEAE sieve-tubes were first discovered by 

 J. Klein, and subsequently described by Wille (Lophura tenuis, 

 Rhytijjhloca pinastroides, Helicothamium scorpoides, Cystocloniam pur- 



