ON FORMATIVE INDUCTION AND PECULIARITIES OF CELLS 141 



under special conditions prove to be labile, as when a root-apex is converted 

 into that of a shoot. Automatic alterations of the previous conditions 

 must also occur when a vegetating plant begins to flower. In fact a 

 particular disposition, whether inherited or subsequently induced, is stable 

 only so long as it is not acted on by any influence capable of reversing 

 or modifying it. 



SECTION 41. On Formative Induction and the Inherent 

 Peculiarities of Cells and Organs. 



The progress of development is determined by the changing properties 

 of the growing parts, and by the external influences exercised upon them. 

 Hence the course followed is liable to modification until with the cessation 

 of growth the power of morphological response is lost. Thus buds may be 

 induced to develop into long shoots or into dwarf spurs according to the con- 

 ditions ; the leaf-primordia of aquatic plants may similarly be caused to form 

 either aquatic or aerial leaves, while the primordia QiPrunus y Padus, and other 

 trees, which normally develop into bud-scales, may form foliage leaves when 

 the buds which would rest over winter are caused to undergo direct develop- 

 ment by removing all the foliage 1 . Naturally if the bud happened to be 

 partially opened at the time, the oldest primordia take on partially or 

 entirely the character of bud-scales, while the younger ones pass by gradual 

 transitions into foliage leaves. Similar gradual transitions must occur when 

 an induced dorsiventrality is reversed by the changed direction of the 

 orienting stimulus. 



The principles are essentially the same whether we are dealing with an 

 organ of limited growth, such as a leaf-primordium, or with a shoot- apex 

 in which the determining factors may act not only on the developing 

 primordia, but also upon the primary meristem. In each case the extent to 

 which the general powers of the meristem cells are automatically altered or 

 suppressed during the process of differentiation has to be separately deter- 

 mined, as well as the extent to which the observed modification is due 

 to the influence of neighbouring parts. All primordia are not, however, 

 specifically and permanently dissimilar, for otherwise a root-apex could not 

 be converted into that of a shoot. Similarly the leaf-primordia probably 

 possess at first more or less generalized properties, for although they have 

 never been directly converted into root- and shoot-apices, still the reactions 

 resulting from injuries show that the young leaf-primordia possess the 

 general characters of the embryonic meristem. 



Considerable change of shape is possible without the embryonic 

 property being lost, as is shown by the peculiar forms assumed by such 



1 Cf. Goebel, Bot. Ztg., 1880, p. 807. 



