8i7 



The Effect of Constricting the Axis. 



The "Constriction" consists in the close binding of an inelastic band 

 (i. e. string, wire, etc.) about a trunk or branch. The results of this treat- 

 ment show, to the casual observer, that this constriction of the axis is 

 nothing but a local, artificial increase of the sap pressure. But here the 

 most extreme case of sap pressure takes effect at once, since the formation 

 of new structures below the constricting place are gradually reduced to a 

 minimum and finally disappear entirely. The xylem elements, near the con- 

 stricting band, thus deviate from their perpendicular course, even increasing 

 their inclination to the horizontal, so that I think in the different normal 

 trees themselves the more or less spiral twisting of the wood fibres is con- 

 nected with the greater or lesser pressure exerted by the sap. 



Finally, the tree becomes so thick above the constricted place that the 

 bark splits above the band and later also below it. This removes the sap 

 pressure almost entirely. The result is a luxuriant formation of wood 

 parenchyma which, with the aging of the plant part, passes over gradually 

 in the later annual layers into normal wood and overgrows completely the 

 band or the wire. Such an overgrown constriction bears great outward 

 resemblance to a grafted place but has naturally no internal structural resem- 

 blance to it. 



In Fig. 196 (see page 819) two dift'erent stages of the constriction are 

 shown. Fig. 196, / is a year old maple branch, with a constricted place 

 only a few months old. Fig. 196, 2 is an older branch, which shows the 

 overgrowth of a wire ring, several years old. Fig. 196, J is a longitudinal 

 section of Fig. 196, 2, where d and d' represent the cross sections of the wire 

 ring; u represents the overgrowth edge, which is more greatly developed on 

 one side {u) by the increased supply of nutritive substances from the branch 

 {2) above it. Here it has overgrown the wire earlier than on the opposite 

 side. 



An anatomical investigation of the stage represented in Fig. 196, i 

 shows that the constriction at first cannot produce very extensive changes. 

 The bark has suffered the greatest disadvantage and it is chiefly the cell 

 layers, lying on the outer side of the primary bark, between the phloem 

 fibres, or between the stone cell aggregations and the epidermal cells, which 

 have been especially compressed. The cell layers next the phloem fibres 

 seem to be the most pressed together ; the effect is less marked on the next 

 layers toward the outside, which are often thickened like collenchyma. 

 Their cells are compressed to j^ or % their normal diameter and it would 

 seem as if they hereby become somewhat longer than the corresponding 

 cells in an unconstricted place. The sub-epidermal, almost square cells, are 

 compressed to half their diameter. The epidermis suffers least of all. 



If, as in Fig. 196, i, the constricting band is wound several times about 

 the branch, apparently very prominent callus rolls become noticeable 

 between every two turns. In them the aforesaid parts of the bark are devel- 



