640 



THE SUBDIVISIONS OF THE VEGETABLE KINGDOM. 



Hydrodictyon, the Water-net (figs. 370^ and 370^) is a beautiful organism 

 forming net-like colonies of cylindrical cells, which are joined end to end, forming 

 the sides of the polygonal meshes. Each cell may be as much as 1 centimetre in 

 length. A thin layer of protoplasm containing numerous small nuclei lines the 

 wall and incloses a large central vacuole. The chromatophore, or chlorophyll-layer 

 of the protoplasm, contains many pyrenoids, each surrounded by a sheath of starch 

 grains. Fine-grained starch is also scattered through the substance of the chro- 



Fig. 370.— Hj'drodictyacese. 



1 The Water-net {Hydrodictyon utriculosum), nat. size. 2 A portion of the same niagnifled 50 diameters. s_ 4 and 5 Formation 

 of zoospores in a cell of Hydrodictyon, showing their union together, and escape as a young net ; x 300. « Pediastrum 

 granulatum; development and escape of zoospores, the lightly-dotted chambers already vacated. 7 and s Zoospores after 

 their escape arranged as a new Pediastrum plant ; x 240. 



matophore. This stroma-starch appears in great quantity when growth is checked 

 and assimilation remains active, disappearing again if assimilation is stopped. The 

 pyrenoid-starch, on the other hand, seems to be withdrawn from the ordinary meta- 

 bolism of the cell, since it is formed round each pyrenoid early in the life of the 

 cell, and remains there under all circumstances, unless the cell is on the point of 

 actual starvation in the dark, till the onset of reproduction. When zoospores are 

 about to be formed the pyrenoids together with their starch disappear, and abund- 

 ant stroma-starch appears. At the same time the nuclei multiply a good deal by 

 division, and eventually the whole of the protoplasm divides to form a great num- 



