576 MICROSCOPIC FORMS OF VEGETABLE LIFE-THALLOPHYTES 



large glass jars exposed to the light, all that is necessary being to 

 pour off the water occasionally from the upper part of the vessel 

 (thus carrying away a film that is apt to form on its surface), and 

 to replace this by fresh water. Each plant is composed of an 

 assemblage of long tubiform cells placed end to end, with a distinct 

 central axis, around which the branches are disposed at intervals 

 with great regularity (fig. 434, A). In Nitella the stem and 

 branches are composed of simple cells, which sometimes attain 

 the length of several inches ; whilst in most species of Char a each 

 central tube is surrounded by an envelope of smaller ones, which is 

 formed as in jBatrachospermum, save that the investing cells grow 

 upwards as well as downwards from each node, and meet each other 

 on the stem halfway between the nodes, their ends dovetailing 

 into one another. These investing tubes constitute what is termed 

 the * cortex ' of Chara. They are of smaller diameter than the central 

 tube, and are arranged spirally round it, giving the stem a twisted 

 appearance. Each ' node,' or zone from which the branches spring, 

 consists of a single plate or layer of small cells, which, in Chara, are 

 a continuation of the cortical layer of the ' internode.' The branches 

 are altogether similar in structure to the primary axis, and 

 terminate in a large elongated pointed cell, which is not covered by 

 the cortex. From the lower part of the stem ' rhizoids ' or rooting 

 filaments are put out, which attach the plant to the soil. Some 

 species have the power of secreting carbonate of lime from the 

 water in which they grow, if this be at all impregnated with 

 calcareous matter ; and by the deposition of it beneath their tegu- 

 ment they have gained their popular name of ' stoneworts.' The 

 long tubiform cells of Nitella, and the terminal uncorticated cells of 

 the branches of Chara, afford a very beautiful and instructive display 

 of the phenomenon of cydosis, or rotation of protoplasm in their 

 interior. Each cell, in the healthy state, is lined by a layer of 

 chlorophyll grains, which cover every part, except two longitudinal 

 lines that remain nearly colourless (fig. 434, B) ; and a constant 

 stream of semi-fluid protoplasm, containing starch grains and 

 chlorophyll granules, is seen to flow over the green layer, the 

 current passing up one side, changing its direction at the extremity, 

 and flowing down the other side, the ascending and descending 

 spaces being bounded by the transparent lines just mentioned. In 

 the young cells the rotation may be seen before this granular 

 lining is formed. The rate of the movement is affected by anything 

 that influences the vital activity of the plant ; thus it is accelerated 

 by moderate warmth, whilst it is retarded by cold ; and it may be 

 at once checked by a slight electric discharge through the plant. 

 Carried along by the protoplasmic stream are a number of solid 

 particles, which consist of starchy matter, and are of various sizes, 

 being sometimes very small and of definite figure, whilst in other 

 instances they are seen as large irregular masses, which appear to 

 be formed by the aggregation of the smaller particles. The produc- 

 tion of new cells for the extension of the stem or branches, or for 

 the origination of new whorls, is not here accomplished by the 

 .subdivision of the parent-cell, but takes place by the method of out- 



