262 BOTANY PAI:T i 



Since in these tropical localities endemic species may exhibit well-marked periods 

 of rest and of renewed vegetative activity, it is clear that such periodicity is not 

 merely induced and regulated by external influences. Its cause must rather be 

 sought in an autonomous, rhythmic course of the vital process itself. Although to 

 so many plants winter is the season of rest and cessation from growth, other 

 plants, e.g. certain Lichens and Mosses, seem to find in the warmer days of winter 

 the most favourable conditions of vegetation ; and in summer, on the contrary, 

 either do not grow at all or only very little. Similarly, many spring plants attain 

 their highest development, not in summer, but during the variable weather of 

 March and April, and, for the most part, they have entered upon their period of 

 rest when the summer vegetation is just awakening. 



In countries where there are alternate rainy and dry periods, the latter 

 generally corresponds to the period of vegetative rest. 



Favourable conditions may shorten the resting period, especially if tin- 

 organisms have previously been exposed to the influence of frost, drought 

 (withering), or, as JOHANNSEN discovered, to ether vapour, etc. This premature 

 awakening does not, however, result equally readily at all periods of the resting time. 

 It occurs best in the later portion of the latter, just before the normal time of 

 awakening, and almost equally well in the earlier portion just after arrest has taken 

 place. Potato tubers and bulbs in some cases do not completely enter into a 

 resting state before their development is awakened. In the intervening period of 

 complete rest such attempts are usually without result. These facts must l>e 

 considered in the forcing of plants, as must also the fact of the temperature 

 optima differing for the several developmental processes, for some of which tin \ 

 are relatively low ( 7:i ). 



DURATION OF PLANT LIFE. The life of a plant, during the whole 

 of its development, from its germination to its death, is dependent upon 

 external and internal conditions. In the case of the lower vegetable 

 organisms, such as Algae, Fungi, and Bacteria, their whole existence 

 may be completed within a few days or even hours, and indeed some 

 of the higher herbaceous plants last only for a few weeks, while the 

 persistent shrubs and trees, on the other hand, may live for thou- 

 sands of years. 



After the formation of the seeds, there occurs in many plants a cessation of 

 their developmental processes, and such a complete exhaustion of vitality that 

 death ensues. Such an organic termination of the period of life occurs in our 

 annual summer plants, which, according to KLKBS, can be made perennial by 

 artificially preventing their reproduction. It also takes place with plants in 

 which the preparatory processes for the formation of fruit have extended for two 

 or more years, as in biennial plants in the case of the 10- to 40- year-old Agave, 

 which, after the formation of its stately inflorescence, dies of exhaustion, and in 

 some Palms (Corypha umbraculifera). In plants, on the other hand, which in 

 addition to the production of flowers and fruit accumulate also a reserve of organic 

 substance, and, with their reproductive organs, form also new growing points, life 

 does not cease with the production of the seeds. Such plants possess within 

 themselves the power of unlimited life, the duration of which may only be 

 terminated by unfavourable external conditions, the ravages of parasites, injuries 

 from wind, and other causes. 



The longevity of trees having an historical interest is naturally best known 



