31 



Oped stem Is the total absence of lignified tissue in all the inter- 

 nodcs. It is true that in all aquatic phanerogams there is a con- 

 siderable reduction of the vascular tissue, but in related plants, 

 such as NymphcBa and Niiphar, we still find some very distinct 

 tracheal ducts, besides the numerous '' internal hairs," the thick 

 walls of which are decidedly lignified. As in most other aquatic 

 plants, a large portion of the stem is occupied by a number of 

 very large intercellular canals, (Fig. 2, ic), which, on the cross 

 section, have an ovate, oval, or sometimes circular outline. They 

 are separated by partitions consisting usually of one layer of cells 

 only. The cortical portion is made up of three or four rows of 

 more densely arranged, large parenchyma cells, and the epiderm- 

 is of one layer of cells with a well-developed cuticle. Many of 

 the last-named cells bear peculiar hairs, to be described hereafter. 



The central portion of the stem is invariably occupied by two 

 fibro-vascular or mestom bundles (Fig. 2, m), (As there are no 

 vessels nor fibres proper in these bundles, the new term '' mes- 

 tom " is doubly preferable to the old '' fibro-vascular bundle "). 

 The two mestom bundles are separated by parenchymatous tissue 

 and groups of intercellular canals. Each is composed of two 

 large bundles of sieve-tubes with their accompanying cells, (Fig. 

 2, 1), The sieve-tubes are very wide, with finely perforated, ob- 

 lique septa. The sieve or ** leptom V bundles have, on the cross- 

 section, an ovate outline, the pointed ends being directed towards 

 a large circular intercellular canal, that takes the place of the vas- 

 cular or "hadrom" element of the mestom, (Fig. 2, h). This 

 canal, as well as the leptom bundles, are strengthened at their 

 circumference by a row of thick- walled cells. The whole mestom 

 bundle is surrounded by an endodermis which plainly shows the 

 " black dots '' on the radial walls. At any height of a given 

 internode the cross-sections are alike : nowhere could be found 

 any anastomosis between the two separate bundles of an internode, 

 as is the case in related plants. 



When we arrive at the immediate proximity of the node, the 

 large intercellular canals come to an end. The peripheral ones 

 are closed with a spongy tissue of stellate cells, through the meshes 

 of which the canals below the node are in open communication 

 with those of the next internode. In the central intercellular canal 



