Chap. Ill] FRESH-WATER VEGETATION 819 



the most important assimilatory organs. Another much rarer property 

 of the roots of many species is that of producing vegetative and fertile 

 adventitious shoots ; they thus play the chief role in sexual and asexual 

 reproduction. On the other hand, forms are not wanting that are 

 entirely devoid of roots. 



The differentiation into stem and leaf is clearly shown in the species 

 figured and is comprehensible without further explanation. Axes and 

 leaves, especially the latter, exhibit great variety, even in such less aberrant 

 forms. The leaves, for instance, in some species are reduced to minute 

 scales, whilst in others they are i\ meters in length. Other species pos- 

 sess a highly peculiar construction, which is caused sometimes by the 

 predominance of roots, sometimes by the coalescence of shoots to form 

 thallus-like plates. 



The young flower-buds first commence to open either when the water 

 sinks, or more frequently when the shoots emerge above the water. Two 

 factors generally favourable to reproductive growth, dryness and light, 

 come strikingly into play. The leaves, no longer covered with water, dry 

 up and quickly fall off, frequently together with part of the shoots and the 

 roots, so that it is only more or less reduced remnants of the vegetative 

 system that bear flowers and fruits (Fig. 501, 4 and 8). The flowers open 

 only in the air and are pollinated by insects. The minute seeds germinate 

 as soon as they are moistened, and the seedlings very quickly develop 

 contrivances for fixation. 



The Podostemaceae differ anatomically from other aquatic phanerogams 

 in the very weak development of air-containing intercellular spaces which 

 do not surpass those of ordinary terrestrial plants. The rapidly moving 

 water which is rich in air renders the possession of an internal aerating 

 system as unnecessary here, as it is in the case of plants in the moving sea. 

 Further, in contrast with the phanerogams of still water, Podostemaceae 

 possess a well-developed mechanical tissue, which is centrally arranged 

 round the vascular bundles, and affords the tensile strength necessary for 

 such a mode of life. 



4. PERIODIC PHENOMENA OF THE VEGETATION 

 OF FRESH WATER. 



The influence of seasons on vegetable life in fresh water has been investi- 

 gated in detail only in Central Europe. Only a few macrophytes of the 

 benthos and of the hemiplankton are annuals ; for instance, in the former 

 Naias minor and N. flexilis, Subularia, and in the latter Salvinia natans. 

 Most species perennate, and remain some of them apparently unchanged, 

 for example Zannichellia, the submerged species of Batrachium, Vallisneria, 

 whilst others descend to the bottom of the water owing to the injection of 



3G 2 



