=295. Simplicity of the process.=—In spirogyra any cell of the thread may form a gamete (excepting the holdfasts of some species). Since all of the cells of a thread are practically alike, there is no structural difference between a vegetative cell and a cell about to conjugate. The difference is a physiological one. All the cells are capable of conjugation if the physiological conditions are present. All the cells therefore are potential gametes. (Strictly speaking the wall of the cell is the _garnetangium_, while the content forms the gamete.) While there is sometimes a slight difference in size between the conjugating cells, and the supplying cell may be the smaller, this is not general. We say, therefore, that there is no differentiation among the gametes, so that usually before the protoplasm begins to move one cannot say which is to be the supplying and which the receiving gamete. =296. Position of the plant spirogyra.=—From our study then we see that there is practically no differentiation among the vegetative cells, except where holdfasts grow out from some of the cells for support. They are all alike in form, in capacity for growth, division, or multiplication of the threads. Each cell is practically an independent plant. There is no differentiation between vegetative cell and conjugating cell. All the cells are potential gametes. Finally there is no structural differentiation between the gametes. This indicates then a simple condition of things, a low grade of organization. =297.= The alga spirogyra is one of the representatives of the lower algæ belonging to the group called _Conjugatæ_. Zygnema with star-shaped chloroplasts, mougeotia with straight or sometimes twisted chlorophyll bands, belong to the same group. In the latter genus only a portion of the protoplasm of each cell unites to form the zygospore, which is located in the tube between the cells. [Illustration: Fig. 132. Closterium.] [Illustration: Fig. 133. Micrasterias.] [Illustration: Fig. 134. Xanthidium.] [Illustration: Fig. 135. Staurastrum.] [Illustration: Fig. 136. Euastrum.] [Illustration: Fig. 137. Cosmarium.] =298.= The desmids also belong to the same group. The desmids usually live as separate cells. Many of them are beautiful in form. They grow entangled among other algæ, or on the surface of aquatic plants, or on wet soil. Several genera are illustrated in figures 132-137. CHAPTER XV. VAUCHERIA.