INJURIES INDUCED BY PLANTS 



PHANEROGAMS 



No sharp boundary can be drawn between parasites and 

 plants that are only indirectly injurious owing to their proximity, 

 or to their competition for food-materials, light, and so forth.* 

 One gradually passes from the latter to plants which, while 

 not subsisting on the substance of others, still directly attack 

 them, and induce pathological phenomena. 



Allusion may be made, for instance, to Lonicera Periclymenum^ 

 whose stem, when opportunity offers, winds round young trees 

 with the result that a few years later the descent of the plastic 

 materials in the bast is forced to take place in a definite spiral 

 course. With increasing thickness the tree is soon subjected 

 to direct pressure by the twining plant, which prevents the direct 

 vertical descent of the solutions of food-substances coming 

 from the leaves. It frequently happens that the part of the 

 stem immediately below that of the honeysuckle is deprived of 

 nourishment, the result being that the cambium in that region 

 gradually dies ; whereas the portion above the passively tighten- 

 ing stem of the honeysuckle on the one hand exhibits very 

 vigorous growth, and, on the other, experiences abnormal altera- 

 tion in its younger parts owing to the spiral direction imparted 

 to all the organs of the vascular bundles. 



While there is no doubt that the immediate cause of the 

 movement of plastic substances in the cortex and bast is the fact 

 of their being produced in one place and utilised in another, neces- 

 sitating a transference from the place of origin to the place of con- 

 sumption, still the assumption that the movement of the plastic 

 materials in the bast takes place much more easily and quickly 

 in a vertical than in a lateral direction receives support from 

 the fact just mentioned, and illustrated in Fig. I, as well as 

 from many other phenomena. In fact, lateral movement is 

 so difficult as sometimes to induce complete cessation of the 



* [In such cases the plants not valued are regarded as " weeds " ; but it is 

 obvious that any plant may act as a weed towards others, and a little reflec- 

 tion shows that it may act as a weed towards its own species if crowded, 

 &c. ED.] f [Honeysuckle. ED.] 



