238 SENESCENCE AND REJUVENESCENCE 



buds arise in a definite sequence and space relation to each other, 

 and numerous experiments have demonstrated that the growing tip 

 of the main axis which is often itself a complex of young buds 

 dominates to a greater or less extent the whole stem. In the root 

 system somewhat similar relations exist between the growing region 

 of the main axis and the lateral branches. In such plants also 

 the relation between the spatial boundaries of the individual and 

 the development of conducting paths is clearly apparent. Various 

 facts indicate that in vascular plants transmission of stimuli takes 

 place more rapidly and to greater distances along the vascular 

 bundles than through other tissues (see Fitting, '07), and some 

 botanists have regarded the sieve vessels as the chief conducting 

 elements. In the apical growing region of the main axis, where the 

 tissues are embryonic and vascular bundles have not yet developed, 

 new buds, i.e., new phytoids, often arise at very short distances from 

 each other in spite of the high rate of metabolism in this region, while 

 farther down the stem, where the vascular bundles have differen- 

 tiated, the dominance of a growing tip may extend over much greater 

 distances and the dominance of the whole growing region at the 

 apical end of the main axis may extend over the whole length of 

 the stem. 



The prothallia of the liverworts and ferns apparently are single 

 plant individuals, at least during their earlier stages, and some 

 throughout hfe, with a dominant growing region possessing the 

 highest rate of metabolism and a metabolic gradient along the 

 axis. But even these prothallia in many cases show agamic 

 reproduction after a certain stage is reached or under certain 

 conditions. 



Many botanists regard the formation of new growing tips in the 

 vegetative life of the plant merely as growth, and reserve the 

 term "reproduction" for the specialized forms of reproduction, 

 such as the formation of spores, gemmae, and other reproductive 

 bodies, including the gametes. Actually, however, each new 

 growing tip represents a new individuation with the potentiahties 

 of a whole plant and fails to produce a whole plant only because 

 it is organically connected with other parts. Properly speaking, 

 then, the formation of new growing tips or buds is a reproductive 



