WHEN TREES GROW 



1389 



The trees apparently rest but in reality they are preparing 

 for the next upward thrust which may be longer than the 

 original advance. Furthermore, the writer believes that 

 the recurring rest periods may become a rather fixed and 

 regular feature of the growth of certain species. This 

 is certainly true in the case of normal young Pitch Pine 

 in the vicinity of Mont Alto which exhibits annually 



AFTER HEIGHT GROWTH HAS CEASED 



Immediately following the completion of height growth the twigs of Nor- 

 way Spruce assume an erect position, hegin to stiffen, and develop winter 

 buds. 



a cessation of growth for a period of two to three weeks. 



The rate of tree growth not only fluctuates throughout 

 the growing season but also during each day. The maxi- 

 mum growth usually occurs late at night, apparently after 

 the preparation and translocation of food and other 

 essential materials becomes less active, and the minimum 

 growth falls in the afternoon of each clear day when 

 the greatest activity in the manufacture of starch and 

 sugar is in operation. 



About 20 trees of each of the four species given in the 

 following tabulation were measured regularly at 7.30 

 !'. M. and 7.30 A. M. for a specified period. The derived 

 results for height growth during the day and at night 

 are given in the following tabulation: 



SPECIES DAY NIGHT 



Tree of Heaven 35% 65% 



Tulip Tree 40% 60% 



Norway Spruce 18% 82% 



White Pine 39% 61% 



Average 33% 67% 



This tabulation shows that trees grow about twice as 

 much at night as during the day. By using instruments 

 of greater precision the percentages would no doubt be 

 changed somewhat, but the general comparative rate of 

 growth would still stand unchanged. 



To some persons it may appear that the problem of 

 growth behavior of trees has only an academic appli- 

 cation. This point of view is, however, untenable for 

 there is an economic side to the study. If conducted in 

 a scientific manner it will supply the basic data for the 

 preparation of a rational schedule for transplanting in 

 the nursery and setting out trees in the woodlot and 

 forest. Foresters, silviculturists, and plant physiologists 

 recommend that planting and transplanting operations 

 should be conducted when the material to be planted is in 

 a dormant condition. No fault can be found with their 

 recommendation, but in order to execute it properly one 

 must know when trees really are dormant. This can 



A "DOUBLE-HEADER" OF HEIGHT GROWTH OF CHESTNUT OAR 



Height growth often proceeds by leaps separated by rest periods of var- 

 iable duration. The original growth of the season bears mature leaves, 

 while the second period of growth is characterized by a sparse setting 

 of immature leaves. 



be ascertained best by determining when trees grow, since 

 growth is so evident and measurable, and whenever trees 

 are not growing they are dormant, that is, in a static con- 

 dition, the duration of which is hard to determine. 



Furthermore, such a study facilitates the preparation 

 of a schedule for field work covering the problem of 

 growth. That determination of the quantitative and 



