436 THE MICROSCOPE. 



by its leaves growing in threes round a slender stringy stem. The 

 watermen on the river have already named it " Water-thyme," from a 

 faint general resemblance which it bears to that plant. 



In 1851 the Anacharis was noticed by Mr. Marshall and others in 

 the river at Ely, but not in great quantities. Next year it had in- 

 creased so much, that the river might be said to be full of it. 



The colour of the plant is deep green ; the leaves are nearly half an 

 inch long, by an eighth wide, egg-shaped at the point, and beset with 

 minute teeth, which cause them to cling. The stems are very brittle, so 

 that whenever the plant is disturbed fragments are broken off. Its 

 powers of increase are prodigious, as every fragment is capable of 

 becoming an independent plant, producing roots and stems, and 

 extending itself indefinitely in every direction. Most of our water- 

 plants require, in order to their increase, to be rooted in the bottom or 

 sides of the river or drain in which they are found; but this is inde- 

 pendent altogether of that condition, and actually grows as it travels 

 slowly down the stream after being cut. The specific gravity of it is 

 so nearly that of water, that it is more disposed to sink than float. A 

 small branch of the plant is represented, with a Hydra attached to it, at 

 page 196. 



Mr. Lawson has pointed out the particular cells in which the current 

 or circulation may most readily be seen viz. the elongated cells around 

 the margin of the leaf and those of the midrib. On examining the leaf 

 with polarised light, these cells, and these alone, are found to contain a 

 large proportion of silica, and present a very interesting appearance. 

 A bright band of light encircles the leaf, and traverses its centre. In 

 fact, the leaf is set, as it were, in a framework of silica. By boiling the 

 leaf for a short time in equal parts of nitric acid and water, a portion 

 of the vegetable tissue is destroyed, and the silica rendered more dis- 

 tinct, without changing the form of the leaf. 



VASCULAR TISSUE. 



This tissue in plants is somewhat analogous to the vascular system 

 of animals ; for this reason, and also for that of its tubular appear- 

 ance, it was called by the older botanists trachece. It consists of rounded, 

 square, columnar, and elongated tubes or cells, with membranous walls, 

 having spiral fibre within. In some cases many fibres are seen run- 

 ning in the same direction, forming a band, when they are termed com- 

 pound-spiral. Spiral fibres are represented in Plate XVI., No. 21. 

 Under this head other membranous tubes are included, in which the 



