SOME ASPECTS OF PERIODICITY IN PLANTS 71 



Abies lowiana 



Thus in main features the behaviour of each species remained con- 

 stant, Finns Pinaster being earlier in its growth and attaining a 

 maximum in June, Abies lowiana showing a maximum in July. 



In regard to dicotylous trees Christison observed that the 

 buds always commenced to swell and, except in the case of 

 Ulmus, to open before growth in thickness began in the trunk. 

 It is necessary, however, to avoid forming the conclusion from 

 these facts that there is a difference in the time of awakened 

 activity of these two forms of growth, inasmuch as the cambium 

 becomes active in twigs before it awakens in the trunk. Christi- 

 son gave no particulars as to the season of elongation of 

 dicotylous tree-stems, but we know that a number of these 

 are capped by resting buds or terminal scars relatively early 

 in the vegetative season (for instance, I have found even the 

 lime regularly in this condition at the middle of June). 



In regard to growth in length of stems and the lengths of 

 the successive internodes, a number of interesting problems 

 concerning the effect of the innate rhythm on the response to 

 external influences appear not to have been investigated. 

 For instance, if a growing stem destined to attain a certain 

 length under normal circumstances be subjected at a certain 

 phase to an inhibiting influence (say lack of water), will it, if 

 once more provided with favourable conditions, attain its 

 approximate full length by change in length or number of the 

 succeeding internodes ? What are the relative effects on 

 growth in length of the same modifying influences applied at 

 different phases? In general what influence has the correlation 

 between the successive internodes on the nature of the response 

 to external change ? 



It has been shown that the seasons of growth in length and 

 of thickness of a stem do not synchronise in period ; the same 

 is true of the seasons of elongation of the leaves and of the 



