INFLUENCE OF ENVIRONMENT ON PLANT LIFE. 107 



peratures, follow one another in unending succession. In certain 

 periods foliage is produced and growth proceeds with rapidity; in 

 others the leaves disappear and growth is retarded or checked; in 

 still others nearly all activities cease and the branches remain dormant 

 till the return of conditions favorable to growth. 



In a manner similar to the above, we may consider all perennial forms 

 that maintain themselves or spread by vegetative propagation, as in 

 the case of those with underground branches, rhizomes, stolons, divid- 

 ing-bulbs, and the like. In a form like Solomon's seal (Polygonatum) , 

 vegetative propagation is very important ; a single rhizome may give rise 

 to numerous new growing-points (by means of branching) , which eventu- 

 ally become separated from the parent rhizome with the decay of the 

 connecting portions, so that, if time suffices, a considerable area may 

 be occupied without other activity than this purely vegetative one. 



Of course bulbs, tubers, and the like are to be regarded as plant 

 phases which are characterized by dormancy and highly resistant 

 properties, for which the physiological limits are widely separated. 

 There occur all gradations in stem and bud vitality between the bulb 

 or tuber and the great deciduous or evergreen tree, all agreeing in 

 the essential point that with the coming of adverse conditions they 

 undergo a check in their activities. The deciduous forms lose a large 

 proportion of all of their vegetative organs. All these forms remain 

 alive but dormant till the return of the growing-season. 



A second example may be taken from those tropical forms which 

 reproduce by seeds, but in which vivipary vaults the resting-period 

 usually so characteristic of the seed. A history of the activities in the 

 mangrove, for instance, might run somewhat as follows : In the mature 

 phase of this plant there are manifested cell activities which result 

 in the dormancy of many cells. Some of the dormant cells, the eggs, 

 are capable of resuming growth in size under certain conditions, the 

 main condition being the entrance of protoplasmic material from 

 another cell; that is, the occurrence of fertilization. If fertilization 

 takes place, and this is to be looked upon as merely one of the environ- 

 mental changes which act upon the plant in its cycle of developmental 

 phases, then the ovum develops into an embryo and continues growth 

 without any marked pause or resting-stage, forming ultimately a new 

 plant. External conditions must furnish stimuli by changing in quality 

 or intensity at various times in this development, one of the most 

 special of which is the falling of the germinated seed into the mud below. 

 We see in such forms all the usual phenomena of production by seed, 

 but the pronounced dormant phase of the majority of seeds is omitted. 

 However, unless the embryo root reaches the mud of the substratum 

 (which signifies pronounced environmental change) the cycle of 

 growth is checked. 



A third example may be chosen from among the plants which 

 reproduce through seeds, but in which parthenogenesis bridges the 

 interruption which usually precedes the formation of the^ embryo 



