GREEN ALG^E. 39 



leaves and stem, we will examine it in the former, as by cutting several 

 cross- sections of the whole bunch of young leaves near the top of the 

 plant, we shall pretty certainly get some sections through a joint. The 

 arrangement is shown in Figure 23, E. 



As the stem grows, a covering is formed over the large internodal 

 cell (y) by the growth of cells from the nodes. These grow both from 

 above and below, meeting in the middle of the internode and completely 

 hiding the long axial cell. A section across the internode shows the 

 large axial cell (y) surrounded by the regularly arranged cells of the 

 covering or cortex (Fig. 23, D). 



All the cells contain a layer of protoplasm next the wall with numerous 

 oval chloroplasts. If the cells are uninjured, they often show a very 

 marked movement of the protoplasm. These movements are best seen, 

 however, in forms like Nitella, where the long internodal cells are not 

 covered with a cortex. In Chara they are most evident in the root hairs 

 that fasten the plant to the ground. 



The growth of the leaves is almost identical with that of the stem, 

 but the apical growth is limited, and the apical cell becomes finally very 

 long and pointed (Fig. 23, (7). In some species the chloroplasts are 

 reddish in the young cells, assuming their green color as the cells ap- 

 proach maturity. 



The plant multiplies non-sexually by means of special 

 branches that may become detached, but there are no non- 

 sexual spores formed. 



The sexual organs have already been noticed arising in pairs at the 

 joints of the leaves. The oogonium is formed a*bove, the antheridium 

 below. 



The young oogonium (F, 0) consists of a central cell, below which 

 is a smaller one surrounded by a circle of five others, which do not at 

 first project above the central cell, but later completely envelop it (G). 

 Each of these five cells early becomes divided into an upper and a lower 

 one, the latter becoming twisted as it elongates, and the central cell later 

 has a small cell cut off from its base by an oblique wall. The central 

 cell forms the egg cell, which in the ripe oogonium (L, 0) is surrounded 

 by five, spirally twisted cells, and crowned by a circle of five smaller ones, 

 which become of a yellowish color when full grown. They separate at 

 the time of fertilization to allow the spermatozoids to enter the oogonium. 



The antheridium consists at first of a basal cell and a terminal one. 

 The latter, which is nearly globular, divides into eight nearly similar cells by 



