338 



BOTANY 



Characeae, which include six genera and about 160 species, grow in 

 fresh or brackish water, attached to the bottom and covering extended 

 areas with a mass of vegetation. In some species their cylindrical main 

 axes are over a foot in length, and are composed of long internodes 

 alternating with short nodes, from which short, cylindrical branches 



are given off in regular whorls with a 

 similar structure but of limited growth 

 (Fig. 263). The lateral axes are either 

 unbranched or give rise at their nodes to 

 verticillate outgrowths of a second order. 

 From the axil of one of the side branches 

 of each whorl a lateral axis resembling the 

 main axis is produced. The attachment 

 to the substratum is effected by means 

 of branching rhizoid outgrowths from the 

 nodes at the base of the axes. 



Both the main and lateral axes grow 

 in length by means of an apical cell, from 

 which other cells are successively cut off by 

 the formation of transverse walls. Each 

 of these cells is again divided by a trans- 

 verse wall into two cells, from the lower of 

 which a long, internodal cell develops with- 

 out further division ; while the upper, by 

 continued division, gives rise to a disc of 

 nodal cells, the lateral axes, and also, in 

 the lower portion of the main axis, to the 

 rhizoids. In the genus Nitella the long 

 internodes remain naked, but in the genus 

 Chara they become enveloped with a cor- 

 tical layer consisting of longitudinal rows 

 of cells which develop at the nodes from 

 the basal cells of the lateral axes. 



As a result of the fragmentation of its 

 original nucleus, each internodal cell is 

 provided with a number of nuclei which lie embedded in an inner and 

 actively moving layer of parietal protoplasm. Numerous oval chloro- 

 plasts devoid of pyrenoids are found in the internodal cells, disposed 

 in longitudinal rows immediately beneath the cell walls. 



Asexual reproduction by means of swarm-spores or other spores is 

 unrepresented in the Characeae. Sexual reproduction, on the other 

 hand, is provided for by the production of egg-cells and spermatozoids. 

 The female organs are egg-shaped. They are visible to the naked eye, 

 and, like the spherical red-coloured antheridia, are inserted on the nodes 

 of the lateral'axes. With the exception of a few dioecious species, the 

 Characeae are monoecious. 



Fig. 283.— Chara fragilis. End of 

 main shoot. (Nat. size.) 



