322 THE MICROSCOPE. 



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 independent 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, in a subsequent chapter. 



Mr. Lawson pointed out the particular cells in which 

 the current or circulation will be most readily 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 distinct, without changing the form 

 of the leaf. 1 



It is necessary to make a thin section or strip from the 

 leaf of Vallisneria, for the purpose of exhibiting the circu- 

 lation in the cells, as shown in fig. 170, No. 4. Among the 

 cells granules, a few of a more transparent character than 

 the rest, may be seen, having a nucleolus within. 



The phenomenon of cell rotation is seen in other plants 

 besides those growing in water. The leaf of the common 

 plantain or dock, Plantago, furnishes a good example ; the 

 movement being seen both in the cells of the plant, and 

 hairs of the cuticle torn from the midribs. The Spider- 

 wort will be noticed further on. 



(1) See also a paper in Vol. IV. Microscopical Journal of Science, on the Ci* 

 culatiou in the Leal of Anacharis, by Mr F H. Wenham. 



