REVERSION TO THE JUVENILE FORM 171 



the formation of its organs, and especially in the formation of its leaf, 

 simply an arrest which is probably a consequence of relationships of 

 correlation. 



REVERSION TO THE JUVENILE FORM. 



Special interest attaches to the fact that many plants are able to 

 return to their juvenile form, and this we call a ( reversion ' in the 

 ontogenetic sense. At the outset it must be pointed out that these 

 phenomena of reversion take place differently even in species of the 

 same genus ; they may appear in one species under definite conditions, 

 whilst they will not do so in another, and they are sometimes limited 

 to certain regions of the plant-body, or to a definite stage of develop- 

 ment, which once passed, the capacity for ' reversion ' is lost. The 

 experimental treatment of this question has been only recently begun l 

 and should furnish many valuable results. It has in the first place 

 shown in a number of instances that reversion to a juvenile form chiefly 

 takes place when the conditions of vegetation are unfavourably influenced. 

 We have already seen an example of this in the ferns (page 152). 

 Plants with this capacity behave in some degree like hybrids which have 

 two kinds of ' blood ' ; the peculiarities of the parents are usually mixed 

 in the hybrid but they may also appear separately, as in the hybrids of 

 many Cacti. Similarly in many malformations of organs and I mention 

 them simply by way of comparison we note frequently ' reversions ' to the 

 normal form from which they sprang. 



Some Bryophyta supply examples showing that the possibility 

 of reversion is associated with a definite developmental stage. The 

 phenomena of germination in the group will be, as I have already said, 

 depicted in the special part of this book, here I only refer to them 

 from the general stand-point. The Marchantieae produce in germination 

 at first a germ-tube, the apex of which develops into a germ-disc out 

 of which the plantlets arise. The primordia of the young plants may 

 be caused to revert to the formation of germ-tubes in Preissia if the 

 intensity of the light be diminished ; but only so long as they have not 

 reached the stage at which the permanent vegetative point is formed from 

 which the construction of the higher anatomical differentiation of the 

 plant takes place. The cell-mass which in Funaria hygrometrica arises 

 upon the thread-like protonema as the primordium of a moss-bud can, 

 in like manner, be caused to grow out into protonema-threads 2 , but here 

 also only up to a certain age, that namely of the appearance of the three- 



1 Goebel, Uber Jugendformen von Pflanzen und deren kiinstliche Wiederhervorrufung, in Sitznngs- 

 ber. der k. bayer. Akad. d. Wissensch., xxvi (1896), p. 447. 



2 Goebel, 1. c. 



