108 ENVIRONMENTAL CONDITIONS. 



from the mature egg. Here no external change is apparently required 

 to induce the ovum to proceed to the embryonic phase, but the embryo 

 finally reaches a resting-stage, where growth activities are checked. 

 Other alterations in the internal conditions of the dormant phase 

 which we term a seed often require a prolongation of the resting- 

 period, but, in any event, before active growth in size is again manifest 

 definite changes in the external complex are required; the seed must 

 absorb water to a certain degree, the temperature and rate of oxygen- 

 supply must fall within the limits for germination, and various other 

 conditions must be fulfilled in order that the embryo may emerge from 

 its dormant state. 



The dormant phase of the mature egg is omitted or very much 

 reduced in parthenogenesis, that of the seed in vivipary. The periods 

 of rest, or of internal conditions adverse to growth, often coincide with 

 periods of adverse external conditions, and the dormant tissue usually 

 possesses high powers of resistance to the latter. This is a consider- 

 ation the importance of which, in general climatic behavior and 

 distribution of plants, can hardly be overestimated. 



Our fourth and last example is taken from the great majority of 

 plants, where fertilization is necessary and a more or less prolonged 

 period of dormancy intervenes between the maturation of the seed and 

 germination. The annual plant perhaps illustrates this sort of rhyth- 

 mic activity in its simplest form. Germination occurs in the spring, 

 when temperature, moisture-supply, etc., are favorable for this kind 

 of growth. Later, the various developmental phases follow each other 

 with more or less pronounced alterations in the external conditions, 

 and when seeds have been matured the parent plant dies. This final 

 death may occur because of the action of internal conditions, perhaps 

 connected with the ripening of the seed, or because of the action of 

 external factors, such as drought or frost. But the dormant phase 

 represented by the seeds is highly resistant, and these bodies carry 

 the living protoplasm forward through the winter of adverse condi- 

 tions to the beginning of a new cycle of activity. 



The general conception outhned above may be expressed briefly to 

 the effect that each particular sort of plant protoplasm (form, species) 

 is indefinitely perennial, ever passing through repeated cycles, ever 

 changing in internal nature from one developmental phase to 

 another, growing, fragmenting (as in reproduction of all sorts), resting 

 in a dormant condition, and always again taking up the endlessly 

 repeating series. Of course our conception of the repetition here 

 involved must be broad enough to include such alterations from 

 cycle to cycle (variation, mutation, etc.) as the study of evolution 

 demands for the origin of new forms from old. This mode of contem- 

 plating plant activities should be as valuable in physiology and ecology 

 as has been the conception of the alternation of generations in the 

 descriptions of the consecutive steps in plant phylogeny. 



