HO AMERICAN JOURNAL OF BOTANY [Vol. 10, 



taneously with other silk-cotton trees. Why do the "unfavorable external 

 conditions" which cause one silk-cotton tree in Jamaica to rest from leaf 

 production not likewise cause another silk-cotton tree standing near by to 

 rest also? 



Howard's further deduction, that "a plant readily adapts itself to the 

 new demands and the rest becomes a habit," is perhaps applicable to some 

 plants, but not to all. The tropical palm has not yet adapted itself to a 

 temperate climate and acquired the habit of resting in the winter; nor has 

 the temperate-zone apple tree at Tjibodas very successfully adapted itself 

 to a tropical climate after twelve years or more of existence there, even 

 though the climate at Tjibodas is not unlike a temperate summer as regards 

 moisture, temperature, and light. 



One fundamental objection to the belief in a heritable periodicity in 

 bamboos has been raised by several writers. It is pointed out as a remark- 

 able fact that "not only mature clumps but quite slender seedlings" (n, 

 p. 126), "even the buds which have just appeared out of the ground" all 

 blossom at the same time (19, p. 6). This brings us to the consideration 

 of a rather theoretical question, What constitutes age? 



That the parent culms in a large bamboo forest of Dendrocalamus or 

 Chusquea are all of the same age is self-evident. They all sprang from 

 seed sown at the same time, i.e., at the time of the simultaneous death of 

 the individuals of the previous forest. As for the "young" shoots, their 

 age is, from one viewpoint, the same as that of their parents since they 

 arose from the same rootstock. Meristematic cells, in the cambium ring, 

 for example, remain perpetually young, though in years they are older than 

 most of the cells which make up the tree. Old cells become young when 

 lateral shoots are formed from old wood in trees, or when in lower animals 

 limbs are regenerated. If we grant that the morphology and function of 

 cells is dependent on their location in the plant, that is, that there is no 

 such thing as specificity of cells, then all the cells of a bamboo clump, in 

 "young" shoots as well as in "old" culms, are potentially the same; there- 

 fore, all are alike affected by age. Consequently, the determiner present 

 in the germ plasm of the "old" culms which causes them to reach sexual 

 maturity at a definite time is likewise present in the "young" culms which 

 arise from a common rootstock. 



How this innate sexual periodicity of some bamboos came into existence 

 it is impossible to say. Either it must have been established in the past as 

 an acquired habit, or it must be purely the expression of the original physical 

 and chemical make-up of the germ plasm. The nicety with which the 

 life cycle of annuals and the growth rhythm of perennials fit in with the 

 seasonal changes of temperate regions leads one to believe that these periodic 

 vital phenomena have been induced through the ages by climatic condi- 

 tions, with the result that the periodicity has become innate, the habit 

 being more firmly established in some plants than in others. The same 



