ADJUSTMENT TO TEMPERATURE 113 



or cold. The last, however, is the usual task of sexual reproduction 

 or generation. 



The fusion of two sexual plasms or gametes produces a sporo- 

 phore. In the phycophytes the latter is one-celled and is usu- 

 ally spoken of as a resting spore. In all cases the sporophore 

 or some part of it is well protected, to avoid drying out during 

 unfavorable conditions, usually those of winter. It is also stored 

 with food, or contains chloroplasts to enable the protoplasm or 

 plantlet to burst the covering and to obtain a good start when 

 favorable factors return. In some algae and fungi the entire 

 sporophore assumes this role, but in the majority of these, and 

 always among the mossworts and ferns, this task is assigned to 

 the spores produced by it. Among flowering plants it is the seed 

 that serves this purpose. In the following brief discussion of the 

 methods of reproduction, it is not feasible to consider all of these, 

 and only the more important are indicated. 



136. Propagation. Fission is the almost universal method of 

 propagation among the unicellular plants. Budding is merely a 

 kind of incomplete fission, and the internal division of the proto- 

 plasm in forming macrozoogonids is practically fission within 

 the cell wall. After plants became multicellular, however, fission 

 merely increased the number of cells in the plant, except in those 

 cases where the cells fall apart to form conidia. Filamentous 

 forms consequently came to break their threads into pieces or 

 hormogones by the modification or death of a cell. Among the 

 multicellular phycophytes the propagules are usually asexual 

 zoospores, while among the carpophytic fungi the latter have 

 become colorless and aerial, constituting conidia. In both cases 

 they are the direct consequences of growth, and are designed for 

 immediate germination under favorable conditions, as well as to 

 bring about distribution. Conidia, however, have acquired more 

 or less resistance, since a long time may often elapse before they 

 reach proper conditions for germination. 



Among the liverworts and mosses the propagules are usually 

 special bits of the plant body or gametophore, which are called 

 gemma?. These are usually formed upon the thallus or the stem, 

 though they are also developed from other parts. When detached 

 they ordinarily grow at once into new plants. In a few cases 

 specialized leaves may serve the same purpose. The fernworts 

 and ferns have placed their stems underground in most cases. 



