288 TREES: THEIB FORMS AND [Feb., 



more upward posture, till the dome is rounded with erect spray. 

 Now, as all these branches had at one time an upward posture, how 

 are those various postures they have now assumed accounted for ? 

 They are accounted for in this way, that from the day branches 

 incline towards horizontal light, they are acted upon more or less 

 effectively by gravitation. The force exerted upon them by gravita- 

 tion is directly as their size, and inversely as the square of their 

 distance from the earth. This force is aided by the superimposed 

 shade pressing in the direction in which gravitation is drawing. But 

 does shade actually push or repel growth ? Severe experiments are 

 needed to convince us that it does. Into these we cannot at present 

 enter, but this instance may be cited. Two trees are planted side by 

 side ; each is shaded on one side by the other. At some distance from 

 these a third tree is planted in similar circumstances every way, 

 except in the circumstance of reciprocal shade. When they arrive 

 at greatness we discover that the branches on the unshaded sides of 

 the two trees grown close to each other are more largely developed 

 than the branches on any side of the tree grown in solitude. Again, 

 branches, in proportion to the size and the age of the trees, are larger 

 on the outer sides of trees grown at the margin of a wood, than on 

 any side of trees grown in the open park. These instances seem to 

 indicate what we have proof of otherwise, namely, that shade repels 

 growth, just as certain surfaces reflect light. The pendant posture, 

 therefore, of some branches on the tree under consideration is due to 

 the conjoint action of shade and gravitation. "We must be careful, 

 however, to distinguish between a tree with odd pendant branches and 

 a 'weeping ' tree with all its branches pendant. The two cases are 

 as different as choice and compulsion, and the phenomenon called 

 ' weeping ' is in no way bound up with the results of superimposed 

 shade, but seems due in some degree to internal conditions of growth. 

 The same is true of ' fastigiate ' forms — the reverse of weeping forms. 

 These laws which operate with so great force upon tlie lower branches 

 of trees, also act, though vyith relaxed energy, on the higher branches. 

 Their force is relaxed according: to the hei<?ht of the branches and the 

 lessening of superimposed shade. Hence the posture of branches. 



Generally, the oldest branches droop most ; the youngest are most 

 nearly vertical ; branches of intermediate age assume an intermediate 

 posture. But only generally. Often we discover among the old 

 branches one, grosser than the rest, taking an upward posture. It has 

 not this posture because it is large, but it is large because it has this 

 posture. The presence of such branches would invariably prove con- 

 tortions to the form of a tree were it not that Nature takes every 

 means to preserve balance. Into the gap caused by such a branch 

 departing from the normal posture branclilets rush to fill it, from 



