24 BOTANY. 



other alga), and growing readily in aquaria when once established. They 

 are of very elegant shapes, and the number of cells some multiple of four, 

 usually sixteen. 



The cells form a flat disc, the outer ones being generally provided with 

 a pair of spines. 



New individuals arise by internal division of the cells, the contents of 

 each forming as many parts as there are cells in the whole colony. The 

 young cells now escape through a cleft in the wall of the mother cell, but 

 are still surrounded by a delicate membrane (Fig. 11, E). Within this 

 membrane the young cells arrange themselves in the form of the original 

 colony, and grow together, forming a new colony. 



A much larger but rarer form is the water net (Fig. 11, 6?), -in which 

 the colony has the form of a hollow net, the spaces being surrounded by 

 long cylindrical cells placed end to end. Other common forms belong to 

 the genus Scenedesmus (Fig. 11, F), of which there are many species. 



ORDER II. Confervacece. 



Under this head are included a number of forms of which 

 the simplest ones approach closely, especially in their younger 

 stages, the Protococcacece. Indeed, some of the so-called Proto- 

 coccacece are known to be only the early stages of these plants. 



A common member of this order is Cladophora, a coarse- 

 branching alga, growing commonly in running water, where it 

 forms tufts, sometimes a metre or more in length. By floating 

 out a little of it in a saucer, it is easy to see that it is made up 

 of branching filaments. 



The microscope shows (Fig. 13, A) that these filaments are rows of 

 cylindrical cells with thick walls showing evident stratification. At 

 intervals branches are given off, which may in turn branch, giving rise to 

 a complicated branching system. These branches begin as little pro- 

 tuberances of the cell wall at the top of the cell. They increase rapidly 

 in length, and becoming slightly contracted at the base, a wall is formed 

 across at this point, shutting it off from the mother cell. 



The protoplasm lines the wall of the cell, and extends in the form of 

 thin plates across the cavity of the cell, dividing it up into a number of 

 irregular chambers. Imbedded in the protoplasm are numerous flattened 

 chloroplasts, which are so close together as to make the protoplasm appear 

 almost uniformly green. Within the chloroplasts are globular, glistening 



