CORRELATIONS 329 



obvious rudiment, or it may be formed in the first instance from the callus. 

 Many transitions occur between these types which are difficult to separate in 

 practice. It is obvious, however, that the plant need not take the trouble to form 

 an absolutely new organ if in the vicinity there exist already the rudiment of one 

 identical with that it has lost. Thus, if the apex of a primary root be removed, the 

 nearest lateral root takes on the function of chief root, and in the case of the stem 

 there are plenty of buds which are only waiting their chance to take on the duty 

 of forming the main axis. Generally speaking the basal lateral buds of a shoot 

 are retarded in growth by the development of buds higher up ; yet frequently 

 in certain trees, such as the lime, the lateral buds high up inhibit the growth of 

 the terminal one, and hence arises the ' sympodial ' mode of growth of this tree. 



When such * reserve organs ' (Goebel, 1903) are absent, a fresh growth of 

 organs takes place from callus. This is of very common occurrence from the 

 stumps of felled trees, where a cambiogenic callus produces numerous buds. 

 Such a proliferation occurs in many plants on amputated portions of roots, 

 and segments of potato tubers are especially capable of forming such shoots. 

 Although in the majority of cases the origin of these buds is endogenous, as is 

 normally the case in the root, there are also cases known where the renovation 

 takes place from epidermal cells. The way in which plants of Begonia are propagated 

 from leaves is well known. If leaves which have had their principal veins cut 

 through be laid on wet sand a callus is formed at the end of each vein, in the 

 formation of which the epidermal cells take part. Growth and division of one of 

 the epidermal cells of the callus gives rise to a new shoot. It ought to be noted, 

 however, that at a certain distance away from the callus uninjured epidermal cells 

 may also give rise to new shoots, so that a normal epidermal cell can, without 

 doubt, retain the power of developing buds. We therefore see that callus forma- 

 tion is by no means an essential antecedent to the development of shoots. 



Roots also may arise after injuries to roots, stems, or leaves, either 

 from rudiments of such already present in these situations or as entirely fresh 

 formations. On the other hand, the plant has, as a general rule, no power 

 of replacing leaves or parts of leaves. If the blade of a leaf be cut off, the petiole 

 usually dies and is thrown off, and the stem also, when its apex is amputated, 

 frequently dies off down to the node next below. Hildebrand (1898) has, 

 however; drawn attention to a case of genuine renewal of the leaf-apex, and 

 Winkler (1902) and Goebel (1903) have recently investigated the matter more 

 closely. If a leaf-blade of a young plant of Cyclamen persicum be cut off — only 

 in a young plant is such an experiment successful — at a short distance from the 

 wound a development of tissue takes place on both sides of the petiole, and 

 this proceeds to form excrescences which, both in outward form and internal 

 structure, must be described as new leaf-blades. [Goebel does not agree with 

 Winkler's view that a genuine renovation of the leaf -blade occurs in this case ; 

 he holds that all that takes place is a continued growth of the leaf base 

 previously correlatively inhibited. (Compare in this relation, as on all ques- 

 tions of regeneration, Goebel, 1905.)] 



The instances we have quoted of the results of injury to the plant have 

 made us acquainted with a number of cases of correlation. The plant possesses the 

 power of replacing parts which have been lost, and thus cells, tissues, and higher 

 members are formed anew, whose development by normal ontogenesis is impos- 

 sible, since their connexion with the rest of the plant is destroyed. Regenera- 

 tion appears not only in the amputated parts but also in the stocks whence the 

 cuttings have been taken. We cannot avoid the conclusion that the capacity for 

 manifesting vital activities of this sort exists in every cell containing protoplasm 

 and that it is in general kept in abeyance only by the influence of related parts. 



We gain a closer insight into the factors which determine plant formation 

 when we examine more closely the situations where regeneration takes place. If 



