112 PLANT PHYSIOLOGY AND ECOLOGY 



of growth or movement also becomes fixed, and is often exhibited 

 by plants in the absence of the usual causes. 



134. The age of a plant, i.e., the total period of growth, depends 

 very largely upon its size and complexity. A unicellular plant, 

 such as a bacterium or an alga, may pass through its life cycle 

 in a few minutes, for, while it does not die, its individuality is 

 lost in consequence of division into two new ceils. At the other 

 extreme are found the trees, many of which live for centuries, and 

 a few for more than a thousand years. Practically all vascular 

 plants require at least one season to complete their development, 

 and the majority of them live for many years. The death of plants 

 which have but a single period of growth is ordinarily due to 

 unfavorable physical factors, or to the fact that all the tissues 

 of the parent plant have taken the permanent form, leaving no 

 meristem to initiate growth during the following season. Woody 

 plants might well seem to be immortal, except perchance for 

 accidents, but even in them the great accumulation of material 

 sets a mechanical limit to the size that can be attained. The 

 approach of this limit is furthermore hastened by the inevitable 

 decay of the dead tissues of the trunk, resulting in the fall of the 

 tree while growth is still possible. 



135. Reproduction. The earliest division of labor in plants 

 produces a nutritive or vegetative part and a reproductive one. 

 The two are absolutely interdependent: they are connected by 

 growth, which is the result of nutrition and the cause of reproduc- 

 tion, or at least the means by which it is brought about. The 

 simplest case of reproduction is that shown in fission, where the 

 production of two new plants from the parent cell is the direct 

 outcome of growth. JMultiplication must have been originally at 

 least merely a consequence of this process, by which a plant was 

 enabled to continue growth by becoming two. Since growth is 

 characteristic of all plants, reproduction in direct consequence of 

 growth is found in practically all groups of plants. This process 

 is ordinarily termed asexual reproduction or propagation, and 

 the cells or parts by which it is carried on are propagules. The 

 formation of propagules can take place only during the period of 

 growth, and these serve, as a rule, for multiplication under favor- 

 able conditions. A large number of propagules, however, pass 

 into a resting condition by the formation of a protective covering, 

 and thus serve to carry the plant or a portion of it through drouth 



