Fig. 561. A piece of the rose-red Delesseria Leprieurei, double natural size. 562. A piece cut out and much magnified, showing that it is composed of a layer of cells. 563. A few of the cells more highly magnified: the cells are gelatinous and thick-walled.
511. The Grass-green Algæ sometimes form broad membranous fronds, such as those of the common Ulva of the sea-shore, but most of them form mere threads, either simple or branched. To this division belong almost all the Fresh-water Algæ, such as those which constitute the silky threads or green slime of running streams or standing pools, and which were all called Confervas before their immense diversity was known. Some are formed of a single row of cells, developed each from the end of another. Others branch, the top of one cell producing more than one new one (Fig. [564]). Others, of a kind which is very common in fresh water, simple threads made of a line of cells, have the chlorophyll and protoplasm of each cell arranged in spiral lines or bands. They form spores in a peculiar way, which gives to this family the designation of conjugating Algæ.
Fig. 564. The growing end of a branching Conferva (Cladophora glomerata), much magnified; showing how, by a kind of budding growth, a new cell is formed by a cross partition separating the newer tip from the older part below; also, how the branches arise.
Fig. 565. Two magnified individuals of a Spirogyra, forming spores by conjugation; a completed spore at base: above, successive stages of the conjugation are represented.
512. At a certain time two parallel threads approach each other more closely; contiguous parts of a cell of each thread bulge or grow out, and unite when they meet; the cell-wall partitions between them are absorbed so as to open a free communication; the spiral band of green matter in both cells breaks up; the whole of that of one cell passes over into the other; and of the united contents a large green spore is formed. Soon the old cells decay, and the spore set free is ready to germinate. Fig. [565] represents several stages of the conjugating process, which, however, would never be found all together like this in one pair of threads.
