246 XIX. STRUCTURE OF THE THALLOPHYTA. 



is seen a single hemispherical cell, the apical cell, in which is 

 visible a well-defined protoplasmic body, and a rather irregularly- 

 shaped nucleus. In some cases two nuclei will be seen, placed 

 one nearer the flat side of the cell, and the other nearer the free 

 apex ; such a cell is about to divide. The dividing wall is formed 

 parallel to the flat basal wall of the apical cell. In this way cells 

 are cut off from the apical cell. The cell which resulted from the 

 previous division will be seen adjoining the basal wall. Tracing 

 back from the apical cell, it will be seen that the cells become 

 alternately nodal and' internodal cells. 



Char a. Let us now turn to Chara, selecting for the purpose 

 the widely spread C. fragilis. Externally this plant resembles 

 Nitella, but it will be seen upon examination that the internodal 

 cells, instead of being naked, are covered with a layer of cells 

 which we call the cortex, consisting of slender cells which are 

 themselves divided into cell-rows. Similarly, the nodes are flat 

 disks of cells, of which the external ones give rise to the whorls 

 of appendages, and also to the cortical cells. Bhizoids arise from 

 the lower nodes, as in Nitella. The leaves are segmented like 

 the stem, and from their lower nodes produce leaflets. The 

 terminal cell of the leaf has no cortex, and in it the general 

 features of the internodal cell of Nitella are repeated. The bud 

 resembles that of Nitella, excepting that the nodal cells undergo 

 early segmentation by vertical walls, so as to produce a disk 

 consisting of a central and a layer of external cells, these latter 

 .growing out in such fashion as to overlap the undivided inter- 

 nodal cells, and, by subsequent division, produce the cortex. 



Cladophora glomerata. The genus Cladophora consists of 

 abundantly- branched green threads, whose cells decrease in thick- 

 ness with the successive branchings. These are the most widely 

 distributed of all fresh-water algae, and any species is suited for 

 examination. The determination of species is, however, very 

 uncertain in this genus. We select the dark-green, undulating, 

 tufted Cladophora glomerata for examination. This is corymbosely 

 branched, the branches arising, as in all other Cladophorese, from 

 the upper end of the cells. The branching proceeds acropetally, 

 so that the end segments of the branches act as apical cells. 

 Subsequently branches arise also from the older segments, pro- 

 ducing what we may call adventitious shoots. With sufficiently 

 strong magnification, the green peripheral cytoplasmic layer of 

 the cells is seen to be composed of small polygonal plates, the 



