42 GENERAL DIFFERENTIATION OF THE PLANT-BODY 



4. The vegetative points act as attractive centres for plastic material, 

 their influence being stronger or weaker according to their position. If, 

 for example, the vegetative point of the prothallus in a fern be destroyed 

 or cut away, numerous ' adventitious shoots ' spring from the prothallus 

 which could not have arisen so long as the vegetative point was active 

 (see Fig. 20). In a plant with many vegetative points there is there- 

 fore frequently a kind of competition between the vegetative points 

 a phenomenon of which we shall speak when we deal with ' correlation V 



The formation of organs at the vegetative point is the normal 

 relation. Mature cells which have once entered as elements into the 

 construction of a definite portion of a plant-body are incapable of 

 further development if vegetation is not disturbed. They have however 

 frequently not yet lost the capacity of other development, but this usually 

 exists only in a latent condition and is only called forth when the 

 reciprocal influence of the cells is annulled. This is what happens in the 

 phenomena which are commonly grouped as regeneration. I purposely 

 avoid the expression ' adventitious formation ' because very different things 

 are understood by that. When, for example, the buds which appear 

 upon the leaves of many ferns, such as Asplenium bulbiferum and others, 

 and amongst the Spermaphyta in Bryophyllum calycinum, as well as 

 those which are formed upon an old severed leaf of a Begonia, are all 

 of them termed ' adventitious,' such terminology is pointless. The shoots 

 in the first examples arise upon quite the youngest stages of the leaves 

 so long indeed as their tissue retains an embryonal character; they appear 

 at definite places, and belong throughout to the normal course of develop- 

 ment of the plant. If we call these leaf-born shoots adventitious, all that 

 we say is that they are absent from the leaves of most other plants. In 

 the last-mentioned instance, on the other hand, the buds are produced 

 from cells which have already passed into a permanent condition and 

 whose definite peculiarities have been already acquired, and it is only 

 structures such as these that we can call in the strict sense of the word 

 ' subsequent ' or ' adventitious.' The formation of buds in the ferns to 

 which we have referred above is a part of the normal sequence of their 

 formation of organs, just as shoots are regularly produced in progressive 

 serial succession on the roots of Podostemaceae. 



When speaking here of the new formation of organs which takes 

 place on severed parts of plants or upon injured plants as regeneration, 

 I leave out of consideration phenomena of callus-formation, healing of 

 wounds, &c., which belong to the province of anatomy. The relationships 

 of correlation which so often play an important part in regeneration will 

 be dealt with in our Fifth Section. 



1 See the Fifth Section. 



