REPRODUCTION 



105 



Let us consider plants first. Some of them, as every 

 one knows, last only, it may be, a few hours, a few 

 days, or a few months. Others again, and these 

 include all our higher plants, are annual, biennial or 

 perennial. By annual we mean that the plant starts 

 life as a seed in the beginning of the year, grows to 

 maturity and forms flower, fruit and " seed again in 

 the same year, the parent 

 dying off in late autumn or 

 early winter. Biennials, on 

 the other hand, start life from 

 the seed, and in their first 

 year of growth, devote all 

 their energies to attaining 

 full vegetative maturity, at 

 the same time laying aside a 

 surplus for propagative pur- 

 poses, to be employed in the 

 year following, when the flower 

 and fruit are formed. Lastly, 

 the perennial starts life in one 

 year and may grow for several 

 years before it reaches an age 

 at which it is able to flower 

 and fruit. Thereafter it does 

 so, either every succeeding 

 year or intermittently. These 

 three conditions may be expressed diagrammatically 

 as in Fig. 50. 



In the case of the animal the conditions are quite 

 similar. Some of the lower forms live only for a 

 brief period, a few hours or days ; but the majority 

 of animals, including all the higher ones, live for 

 several years, it may be for a hundred or more, 

 although in no case as long as some of the highest 



Annual 

 ~B 3/ennial 

 C Pervimal 

 a t seed; tcuch. 

 oirclt raorew/z 

 one seasons 

 trrourtfi -The, -.- 



itmtinaiion. of \ ,"' 

 l/iecurre in. A t ^~ " 

 J3 !ndictif-e.s Ike. 

 dtaJh oflHt indLivtiu.a.1. 



FIG. 50. Duration of 

 plants. 



