AGE CYCLE IN PLANTS AND LOWER ANIMALS 247 



ability to go through a new course of development and differentia- 

 tion, indicates very clearly that they have become physiologically 

 younger, and, though I know of no observations bearing directly 

 upon this point, no one can doubt that when a differentiated cell 

 dedifferentiates into a growing tip an increase in rate of respiration 

 and other metabolic processes occurs. 



THE RELATION OF THE DIFFERENT FORMS OF AGAMIC REPRODUCTION 

 IN PLANTS TO THE AGE CYCLE 



Most plants exhibit more than one form of agamic reproduction, 

 and in some species, e.g., certain mosses, several different forms 

 occur. But two forms of agamic reproduction are particularly 

 characteristic of nearly all plant species, one the vegetative form 

 of reproduction, often called vegetative growth, in which new vege- 

 tative individuals essentially similar to the old arise by the forma- 

 tion of buds, branches, etc. ; the other the process of spore formation, 

 which usually occurs only in certain regions of the plant body and 

 after a period of vegetative growth. In some cases, as in the rusts, 

 four or five different kinds of spores are produced by a single species. 

 The spore is in general a cell which becomes isolated from the 

 plant body and sooner or later gives rise to a new individual. In 

 some cases this isolation is physiological, in others it is physical. 

 In the algae and fungi, which must be considered before turning to 

 the higher plants, the spores usually develop into individuals like 

 those from which they arose, and the spore may be either a resting 

 or a motile stage between two vegetative generations. Spore for- 

 mation in these plants is essentially a process of complete or partial 

 disintegration of existing individuals into cells, rather than the 

 addition of new individuals as the result of growth, as in vegetative 

 reproduction under the usual conditions. In the alga Ulothrix, for 

 example, any cell of the filamentous, unbranched plant body may 

 break up into zoospores (Figs. 102, 103); in the branching form 

 Vaucheria the terminal region of the branch separates as a multi- 

 nucleate zoospore (Fig. 104). Among the brown algae the spores 

 arise by separation into single small cells of the contents of special 

 organs, the sporangia (Fig. 105). In the fungus Saprolegnia (Fig. 

 1 06) the sporangium is the terminal region of the vegetative body, 



