CONFERVACE^E. 197 



The mode by which the frond lengthens is twofold. Either 

 new cells are continually emitted, as buds, from the apices 

 of the last formed cells ; or else the old cells continually di- 

 vide in the centre. In the first case the frond continues to 

 lengthen by constant additions to its points, and this is truly 

 acrogenous ; in the other it grows equally throughout its 

 whole length. This latter mode of increase is most frequent 

 among the species with simple filaments. The fructification 

 either consists of zoospores formed out of the colouring matter 

 of the cells, and emitted through an aperture formed in the 

 cell-wall ; or else the whole mass of endochrome contained 

 in a cell, and often the whole contents of two cells are con- 

 centrated into a. sporangium or conceptacle, which is deposited 

 in the water on the perishing of the frond. In the Confervea 

 this sporangium is usually formed out of the contents of a sin- 

 gle cell, but is not always lodged in the cell in which it ori- 

 ginates; for in one genus (Tiresias) supplementary cells, are 

 formed at one extremity of the cell furnishing the sporaceous 

 matter, and in these the sporangium is lodged. In the Zyg- 

 nemeac the matter of two cells constantly goes to form the 

 sporangium. This matter either collects in one of the cells, 

 leaving the other quite empty ; or else is deposited in a sup- 

 plementary cell formed in the connecting tube ; or (as in 

 Staurocarpus), where the two cells inosculate without any 

 tube between them, the sporangium, taking the form of a cross, 

 lies partly in one cell and partly in the other. In the Chce- 

 tophoreae the sporangia are lateral and external, developed as 

 buds from the cells of the filament, or they are the enlarged 

 cells of the ramuli. 



By far the larger number of the Confervaceae inhabit fresh 

 water, and are found in all parts of the world wherever water 

 lies stagnant, and wherever it flows. The bright green, 

 glossy threads that float on the surface of ponds and ditches 

 are commonly species of the sub-order Zygnemete, a highly 

 curious and beautiful family, of which there is no marine ex- 

 ample. When young the filaments lie at the bottom of the 

 pool, but as they approach maturity they float to the surface, 

 where they often lie so thickly as to retain within their 

 meshes large bubbles of air, which they have disengaged 

 during the progress of vegetation, and which is in great part 

 oxygen. When shallow water lies for some weeks in sum- 

 mer on the surface of flat land, it often becomes completely 

 filled with the threads of these plants, which by their vege- 



