44 GENERAL DIFFERENTIATION OF THE PLANT-BODY 



from many animals, especially the lower forms of these, is obviously 

 a consequence of the existence in plants of the vegetative point ; for new 

 organs can arise independently upon the vegetative points and the 

 restoration of a severed portion of a leaf, for example, would be of no use 

 to the plant ; but in animals, which possess no vegetative point, the loss 

 of an organ would be a permanent injury were there no power of regenera- 

 tion of the lost part. 



II. NEW FORMATION OF ORGANS IN REGENERATION. 



The cases we have just cited lead up directly to another series 

 in which we do not find restoration of the lost parts but replacement of 

 them by newly formed ones ; the injury here seems to act as a stimulus. 

 An example has been already given above in the case of roots in which 

 the cut is made too far from the root-apex so that regeneration of the 

 vegetative point cannot take place, and then on the cut surface sub- 

 stitution-roots appear. We find this also in seedlings of peas 1 when the 

 root and hypocotyl are cut off below the point of insertion of the 

 cotyledon ; a callus is then formed out of which one or sometimes more 

 roots arise, and in the latter case there is often a malformation through 

 the ' concrescence ' of two or more roots. When the shoot of a seedling 

 plant of this kind is cut off there appears on the surface of the wound 

 in many cases a callus only, in others however one or two shoots spring 

 from it, and the opportunity to observe a similar development is often 

 afforded on stools of Beech, Poplar, and other trees, where a callus arises 

 from the cambium and from it a large number of shoots sprout. 



During their progress to completion as entire plants portions which 

 have been severed from plants very often exhibit the phenomena of 

 ' polarity V which of course existed previously in the uninjured plants 

 and only becomes more apparent in the regeneration. I can only make 

 a brief reference here to these phenomena. 



On a piece of shoot which has been cut off from the parent plant 

 the primordia of roots develop first of all at the root-pole, that is to say, 

 on that portion which is furthest away from the vegetative point of the 

 piece of shoot ; from the shoot-pole shoots proceed. Roots, so far as 

 they are generally capable of regeneration, behave in a contrary manner 3 . 

 Leaves show no polarity ; in them the new formations arise at the leaf- 



1 Vochting, Uber Organbildnng im Pflanzenreich, ii. p. 19. 



2 See Vochting, Uber Organbildung im Pflanzenreich, i and ii ; Sachs, Stoff und Form der 

 Pflanzenorgane. Gesammelte Abhandlungen, ii. 



3 There are certainly some exceptions to this. One, relating to the tubers of Thladiantha dubia, 

 will be noted in a subsequent section, See the paragraphs in the Fifth Section relating to the action 

 of gravity. 



