666 THE SUBDIVISIONS OF THE VEGETABLE KINGDOM. 



groups of cells (c/. similar phenomenon in Diatoms, p. 626). Many Desmids possess 

 the power of locomotion. Their movement is slow and creeping, and although the 

 means by which it is brought about are by no means fully understood, it has been 

 shown to have a connection with the excretion of mucilage, and to stand in relation 

 with light and gravitation. Some forms (e.g. Closterium and Penium, jSgs. 372^ 

 and 372 ") have a curious rotating movement, one end being temporarily fixed while- 

 the other moves up towards the source of light. 



Conjugation takes place as already described, the conjugating cells either 

 coming into direct contact, or putting out short tubes. The conjoined tubes are 

 known as the conjugation-canal. The zygote may be formed either in this or in 

 one of the conjugating cells. The membrane of the zygote consists of three layers, 

 the outer one being sculptured in various ways. In germination, the outer, sculp- 

 tured membrane is burst open, and the protoplasm divides to form 2-8 cells which 

 gradually take on the characters of the adult individual. 



The cells of the thread-forming Desmids (cf. fig. 372 ^2) possess all the characters 

 of the unicellular types, the daughter cells simply remaining together after division 

 (which always occurs in one plane) has taken place. 



Spirogyracece. — This family contains the single genus Spirogyra, many species 

 of which are amongst the commonest Algae in our ponds and ditches. They form 

 green or yellowish-green slimy masses on the mud at the bottom or floating at the 

 surface of the water. Such a Spirogyra-msiss usually contains several species, 

 although the bulk of it is often formed by one. 



The cells of the different species vary much in diameter, from the delicate S. 

 tenuissima which is only about one hundredth of a millimetre across, to S. crassa, 

 which forms coarse threads as much as one seventh of a millimetre thick. Roughly 

 speaking, the broader the cell, the greater number of chromatophorcs it possesses 

 The smaller species possess only one spiral band in each cell (fig. 25a, I), the largest 

 as many as eight or nine. 



Each Spirogyra-cell is a cylinder, in most cases considerably longer than it is 

 broad (though the relation of length to breadth is variable even in one and the same 

 species), with a very delicate layer of protoplasm lining the wall, and a large central 

 vacuole. Eacli chromatophore forms a band embedded in the protoplasm, and 

 twisting spirally round and round the cell at an approximately constant angle. 

 When there are more bands than one they cross each other at regular intervals, 

 forming a beautiful lattice-work. In some species each band makes several com- 

 plete turns in its course dowm the cell (fig. 373); in others, it may be inclined at 

 such a small angle with the long axis that it makes less than a complete turn in the 

 length of the cell. In S. orthospira the bands are practically parallel with the long 

 axis, so that they do not follow a spiral course at all, but form straight bands. 

 Each chromatophore may be isodiametric, or, on the other hand, it may form a 

 flattened band. In the latter case its edges are usually irregularly scalloped. A 

 single row of pyrenoids at larger or smaller intervals is found in each chromato- 

 phore. 



