Class Diatomaphyceæ
(Bacillariales, Diatomaceæ).
Fig. 171a.
A group of Diatoms: c and d, top and side views of the same form; e, colony of stalked forms attached to an alga; f and g, top and side views of the form shown at e; h, a colony; i, a colony, the top and side view shown at k and n, forming auxospores. (After Kerner.)
358b. The diatoms are minute and peculiar organisms believed to be algæ. They live in fresh, brackish, and salt water. They are often found covering the surface of rocks, sticks, or the soil in thin sheets. They occur singly and free, or several individuals may be joined into long threads, or other species may be attached to objects by slender gelatinous stalks. Each protoplast is enclosed in a silicified skeleton in the form of a box with two halves, often shaped like an old-fashioned pill box, one half fitting over the other like the lid of a box. It is evident that in this condition the plant cannot increase much in size.
They multiply by fission. This takes place longitudinally, i.e., in the direction of the two halves or valves of the box. Each new plant then has a valve only on one side. A new valve is now formed over the naked half, and fits inside the old valve. At each division the individuals thus become smaller and smaller until they reach a certain point, when the valves are cast off and the cell forms an auxospore, i.e., it grows alone, or after conjugation with another, to the full size again, and eventually provides itself with new valves. The valves are often marked, with numerous and fine lines, often making beautiful figures, and some are used for test objects for microscopes.
The free forms are capable of movement. The movement takes place in the longitudinal direction of the valves. They glide for some time in one direction, and then stop and move back again. It is not a difficult thing to mount them in fresh water and observe this movement.
The diatoms have small chlorophyll plates, but the green color is disguised by a brownish pigment called diatomin. The relationships of the diatoms are uncertain, but some, because of the color, think they are related to the Phæophyceæ.
Class Phæophyceæ.
359. The brown algæ. (Phæophyceæ).—The members of this class possess chlorophyll, but it is obscured by a brown pigment. The plants are accessible at the seashore, and for inland laboratories may be preserved in formalin (2½ per cent). (See also Chapter LVI.)