60 COMPOUND ORGANS OP PLANTS. 



have two filamentous plants of Zygnema, showing the differ- 

 ent stages in the process of conjugation, which are seen from 

 below upwards. Fig. 26 is a portion of the network of Hy- 

 drodioton utriculatum. 



THE LICHENS. 



67. Plants composed of a tissue of cells combined in one 

 plane. — In this case, the cell is multiplied, as before, by divi- 

 sion, which takes place in two or more directions, giving rise 

 to a plane or solid mass of cells, some of which are specialized 

 for nutrition, and some for reproduction, as already explained. 

 Only the simplest forms of these plants consist of a single 

 layer of cells. Most frondose sea-weeds, as well as lichens 

 and liverworts, are made up of several such layers. 



68. The term frond, or, as it is called in the lichens, tlial- 

 lus, is applied to those simple expansions of vegetable matter 

 which are neither leaf nor stem, but combine the appearance 

 and the office of both. 



69. The thalli of lichens are composed wholly of cellular 

 tissue, and are either pulverulent, crustaceous, foliaoeous, or 

 arborescent; these difi'erent forms depending, of course, on 

 the different ways in which the cells are developed and com- 

 bined together. Those portions of the thallus which contain 

 the sporules are called apothecia (from arcoeijxri, a repository). 

 In the lowest forms of lichens, the pulverulent thallus over- 

 spreads the surface of rocks and the bark of trees, in the form 

 of an inert leprous crust, resembling on the rocks stains of 

 color. Sometimes the fructification is included within the 

 crust and depressed below its surface. The cortical layer or 



