356 Morphogenetic Factors 



out most fully is one which was first observed in the development of 

 reaction wood of conifers and later in similar tissue in angiosperms. 

 Students of wood structure have long noticed that horizontal branches 

 of coniferous trees are excentric in cross section (p. 175), with the pith 

 nearer the upper side than the lower. Below the pith is a wedge-shaped 

 sector of wood, reddish in color when freshly cut, and hence often 

 called "rotholz." Since the lower side of a branch is obviously under 

 compression, this was long regarded as the cause for the development 

 of this sort of structure, and it was called "compression wood." Its cells 

 are somewhat shorter than those of normal wood, and the micellae in 

 their walls are less steeply pitched. Such wood is absent, save exception- 

 ally, in vertical axes and thus in the main trunk but develops wherever 

 such an axis is forced to grow at an angle from the vertical, as in a tree 

 bent partly over. Such wood grows somewhat more in length than does 

 normal wood and thus produces considerable longitudinal compression. 

 If a tree grown in an oblique position is sawn across, beginning on the 

 lower side, the saw therefore tends to bind. 



This tissue has now been shown not to be the result of growth under 

 compression but to have a nature and function of much morphogenetic 

 significance. It is now more correctly called reaction wood. Together 

 with the corresponding tissue in angiosperms, it has been studied by 

 various wood anatomists, especially Hartmann (1932, 1942, 1943), whose 

 work has been extended somewhat by Sinnott (1952). In young conifers, 

 especially pines, before the growing season began, these workers tied 

 some of the lateral branches downward and others upward. They also 

 tied the tip of the main axis into a position out of the vertical, sometimes 

 even in a loop. When new shoots of the current year developed from 

 the terminal buds of these various axes, these new shoots tended to have 

 the same direction of growth that they would have had if the shoot out 

 of which they grew had not been fixed in an atypical position (Fig. 16-7). 

 The main axis bent around so that it again grew upward. The lateral 

 branches (in white pine) grew out at an angle of about 70° from the 

 main trunk and thus from the directional pull of gravity. In other words, 

 there was, in a sense, a regeneration of the normal growth pattern of 

 the tree. 



In this process the reaction wood performs an essential function, for it 

 always occurs in such a place that its greater lengthwise growth will tend 

 to bend the new shoot (and the free portion of the old shoot) into a 

 direction which would be normal for it. If a lateral branch has been 

 bent upward, for example, the reaction wood will be on the upper side, 

 for this will push the branch down. Such a change continues until the 

 orientation of the shoot is normal, when reaction wood again develops on 

 the lower side, as in ordinary branches. Its function here seems to be to 



