TUBULAR VASCULAR SYSTEMS 377 



fibrous strands are embedded in this [stelar] parenchyma. The fusion 

 is considerably further advanced in P. luccns and P. gramineus. In P. 

 pcctinatus and P. pusillus, finally, the axile strand is reduced to a 

 central intercellular passage which has taken the place of the water- 

 conducting tissue, surrounded by a wide sheath of leptome. Here the 

 vascular system no longer shows any signs of having been formed by 

 the fusion of several distinct strands, and the same remark applies to 

 the substantially similar axile strands of ZannicheUia, Elodea, Hydrilla, 

 CeratophyUum, Najas (Fig. 149), etc. (Concerning the reduction of the 

 water-conducting tissues in these cases cf. above, p. 323.) 



Schwendener explains this tendency of the vascular system to 

 revert to its simplest condition, in water-plants, on mechanical 

 grounds. The stems of aquatic plants are subjected to continued 

 tension in an upward direction, on account of the large amount of 

 air which they contain, and very often have in addition to contend 

 with the action of water-currents. As, however, most of the plants in 

 question prefer stagnant or slowly running water, their mechanical 

 strength need not be of a very high order, and hence their requirements 

 in this respect are usually satisfied by a centralisation of the vascular 

 tissue. The cortical parenchyma is never in danger of drying up, even 

 if it is far removed from the vascular system, so that on that score 

 also the central disposition of the conducting tissues is entirely free 

 from objection. 



2. The simple tubular vascular system [including the solenostele 1<9a 

 and the dictyostele] which occurs in a large number of Ferns, repre- 

 sents the intermediate link connecting the simple axile strand with the 

 more complex types of vascular structure that prevail among Phanero- 

 gams. In most Ferns the young stem contains a solid axile vascular 

 strand, which expands, as the stem increases in thickness, to form a 

 tubular structure enclosing a parenchymatous pith and itself surrounded 

 by a parenchymatous cortex. Since the pith contains both conducting 

 parenchyma and storage tissue, complete isolation of this region must 

 be avoided ; free communication with the pith and conducting paren- 

 chyma of the petioles is secured by the development, at the insertion 

 of each leaf, of a more or less extensive break or gap in the vascular 

 cylinder, the so-called leaf-gap. If the stem is short and the leaves 

 closely crowded, the tubular stele acquires the form of a perforated 

 cylinder or cylindrical meshwork. From the margin of each mesh, or 

 leaf-gap, the bundles of the leaf-trace ascend obliquely into the petiole 

 (Fig. 150). 



While the above-described arrangement of the vascular system is 

 characteristic of the majority of the Filicales, a number of deviations 

 from the type occur within the phylum ; in some cases, for example, 



