xx] AERATING SYSTEM 257 



cellular spaces, is provided by the Podostemaceae, which form 

 in other respects a highly anomalous group. The members of 

 this family, which we have discussed in Chapter ix, flourish in 

 rapidly moving water, even "at the sides of the waterfalls, with 

 the furious current rushing right over them 1 ." The tissues are 

 found to include no large lacunae (Fig. 80, p. 1 1 8) and it is pro- 

 bably for this reason that these plants are confined to water 

 which, on account of its movement, is necessarily well aerated. 

 That the constitution of the Podostemaceae does actually 

 render them dependent on high aeration of the water, is shown 

 by the fact that, if, owing to a fall in the level of the stream, they 

 are left behind in a stagnant pot-hole, death quickly ensues 1 . 



In ordinary hydrophytes, living in still or slowly moving 

 waters, there must be a liability to asphyxiation in the case of 

 the roots or rhizomes more or less buried in the saturated mud. 

 The elaborate air-system, developed in the long petioles of such 

 plants as the Waterlilies, probably plays some part in obviating 

 this danger. These petioles form the connecting link between 

 the submerged rhizome and the floating leaves, which not only 

 themselves produce oxygen in the process of assimilation, but 

 also have free access to the oxygen of the atmosphere. In many 

 cases, the air-canals traversing elongated organs, such as stems 

 and petioles, are crossed at intervals by diaphragms, which are 

 not, however, air-tight. Their structure is illustrated in Fig. 119, 

 p. 184, which shows phases in the development of the cells 

 forming the partitions that, at every node, cross the stem of 

 Hippuris vulgaris, the Mare's-tail. From these drawings it will 

 be recognised that intercellular spaces occur at the angles of the 

 cells, both in youth and age, so that gases can pass freely. 



Although it seems to be generally agreed that oxygen is 

 conveyed by means of the internal air-passages from the assi- 

 milating organs to other parts of the plant, there is still much 

 obscurity with regard to the nature and causes of the movements 

 of gases in water plants. These movements have been studied 

 more particularly in the Nymphaeaceae. In Nelumbo, for in- 

 i Willis,J. C. (1902). 



A. W. P 17 



