2 CHLOROSPERME^. 



The Chlorosperms are decidedly the lowest or simplest in structure not only of the 

 Algae, but of all plants. A very considerable number of them have the frond composed 

 of a single utricle or cell, and all cell-division in such plants issues in the production of 

 new unicellular fronds. The Orders Diatomacece and Desmidiacece, of which some 

 hundreds of genera, and perhaps thousands of species are now known to Botanists, are of 

 this character. In the whole of these, the frond consists of what may be called a bivalve 

 cell ; the primordial utricle being single while the cellular envelope is divided into two 

 halves by a medial line. When such a cell is about to be multiplied by dividing into 

 two, the two halves of the old cell remain unchanged, and a new growth of two new 

 half-cells originates at each side, along the medial line. While this growth is going on, 

 the old half cells are gently pushed asunder, and when it is completed, a separation takes 

 place, and two new fronds float apart, each of them composed of an old half-cell and a 

 new growth which gradually acquires all the characters of the opposing valve. But the 

 unicellular structure is not confined to such minute atoms as the Diatomaceae, or such 

 imperfect organisms as the Protococcus and its allies. Many of the larger Chlorosperms 

 are essentially unicellular, and in some of these the vegetable cell is found of very much 

 larger size than in any other plants. In Codium, Vaucheria, and Bryopsis single 

 cylindrical cells may be obtained several inches in length, and frequently of consider- 

 able diameter. In Valonia, saccate cells sometimes as large as a walnut and often as large 

 as a hazel nut, are found. Botrydium, a little siphonaceous Alga common on damp 

 ground in Europe, exhibits within the compass of a single branching cell all the ordinary 

 organs of a compound vegetable, as much specialized as is possible within such narrow 

 limits : thus, it has a descending axis or root, an ascending axis or stem, and a vesicular 

 body, within which its spores are developed. In Caulerpa, however, if the frond in that- 

 genus be really constructed by the evolution of a single cell, we have the vegetable cell 

 assuming its highest development and attaining gigantic size. These unicellular (?) 

 fronds are sometimes two feet in length, and excessively branched ; with specialized 

 root, stem, branches, and leaves. 



The ordinary fructification of the Chlorospermeas consists of zoospores, or spores 

 endowed with ciliary motion, which have already been spoken of in the General Intro- 

 duction, (Part I. pp. 13-14). These are usually of very minute size, and are formed 

 within the cells of the frond, by the transmutations of the whole cell-contents. Myriads 

 of moving granules are thus evolved, each of which is pointed at one end, and there 

 furnished with two or four vibratile hairs, which act like oars, and drive the granule 

 through the water. In one instance (Hydrodictyon) the zoospores, whilst still 

 retained within the walls of the mother-cell, arrange themselves into a young frond, 

 which issues from the cell perfect in all its parts. But usually they are dispersed in the 

 water, and swim about until they commence germination. In some cases, a solitary 

 zoospore and that of large size, clothed all over its surface with cilia, is formed from 

 the mass of endochrome of the parent-cell. And often, as in Zygnema and its allies, the 

 spore is the result of the union of the matter of two cells. 



Among the more interesting observations recently made on the development of these 

 Algae, Pringsheim's memoirs on the fertilization of their spores by means of spermato- 

 zoids are specially worthy of notice. Male organs of unquestionable character have 



