GROUP I. THALLOPHYTA : ALGJI : CHLOROPHYCEJ3. 



251 



zoospores, one from each cell, which resemble the zoogonidia, and germinate in 

 a similar manner to form a (potential or actual) gametophyte. 



Series V. CHAROIDEJE. The forms included in this series constitute but a 

 single order, the Characeae. 



Order I. Characeae. The stem is distinctly segmented into nodes and 

 internodes, the nodes being marked by the whorls of leaves which they bear. 

 It consists of a longitudinal series of elongated cylindrical cells, each of which 

 constitutes an internode, separated from each other by transverse plates of 

 small cells which are the nodes. In Chara there is, in addition, a cortex con- 

 sisting of rows of cells, sometimes spirally twisted, produced by a growth of the 

 peripheral cells of each node, both upwards and downwards, over the inter- 

 nodes above and below it. 



All the cells contain small discoid chloroplastids which lie imbedded in the 

 protoplasm immediately beneath the cell-wall. The more internal portion of 

 the protoplasmic layer shows the movement known as cyclosis ; the central 

 portion of the cell-cavity, when the cell is fully grown, is occupied by a large 

 vacuole filled with cell-sap. 

 Each cell contains a single nu- 

 cleus when young ; but the long 

 internodal cells, when old, are 

 found to contain many nuclei 

 produced by the fragmentation 

 of the original nucleus. 



The growth in length of the 

 stem is unlimited, and is 

 effected by means of a hemi- 

 spheiical apical cell (Fig. 180). 

 This cell undergoes repeated 

 division, a series of segments 

 being cut off by transverse 

 walls ; after a segment has 

 been cut off, the apical cell re- 

 gains its normal size by growth, 

 then another segment is cut off, 

 followed by renewed growth, 

 and so on. Each segment is immediately divided into two cells by a transverse 

 wall ; of these two cells the upper, in all cases, becomes a node, dividing by 

 vertical walls into the small cells, central and peripheral, of which the node 

 consists ; the lower, in all cases, becomes an internode ; it does not divide, but 

 simply grows in length. In Chara the young peripheral nodal cells keep pace 

 with the growth of the internodal cells, forming the cortex over them. 



The leaves and branches of the stem are all developed from the cells of the 

 nodes ; the leaves spring in a whorl, one from each of the peripheral cells of 

 the node, and the branches are developed as buds in the axils of one or more 

 of the leaves of each whorl. 



The mode of growth and general morphology of the leaf is essentially the 

 same as that of the main stem or one of its branches ; it grows by means of an 

 apical cell resembling that of the stem, and from the segments are formed 



FIG. 180. Diagram of growing-point of stem of 

 Chara fragilis ( x 500, after Sachs) : a apical cell ; 

 s segment lately cut off; ri 1 n a n 3 successive nodes ; 

 in 1 in 2 iii 3 successive internodes ; I leaves ; c cortical 

 cells growing down over in from n, 3 . 



