60 GEXERAL DIFFERENTIATION OF THE PLANT-BODY 



3. MORPHOLOGICAL IMPORTANCE OF ARRESTED ORGANS. 



The morphological importance of arrested organs lies in the fact 

 that they often supply the clue by which affinities with allied plants can 

 be ascertained, and phylogenetic morphology is accustomed to interpret 

 arrested organs as vestiges of ancestral structures. It is probable, for 

 example, that the flowers of Salvia, which possess at the present day 

 two complete and two arrested stamens, took origin from a form which 

 like other Labiatae possessed four developed stamens. 



We must however guard against considering all arrested organs as 

 being descended from organs which were developed in the ancestors of 

 the existing forms. Such a view would be a mere chimaera in the case 

 of the regularly arrested flowers of many inflorescences. The assumption 

 is much more probable in their case, as I have already endeavoured 

 to prove in connexion with the inflorescences of grasses 1 , that the 

 plastic material which is present in an inflorescence suffices indeed for 

 the laying down but not for the unfolding of a great number of organs, 

 and this may have been the case from the first in any of our existing 

 forms. It is indeed a quite general rule that many more primordia of 

 organs are formed than become functional, and this failure of function 

 is brought about, as in the cases that have just been mentioned, either 

 by an early arrest of the primordia of the organs or by the withering 

 of the completely formed organs. 



Most of our phylogenetic series are reduction-series, that is to say, 

 are those in which the changes are brought about by arrest. There is 

 a simple psychological explanation of this. If we have a definite ' type ' 

 we obtain through it a fixed starting-point for our comparison. But this 

 is wanting when our comparisons deal with an ascending and not a 

 descending series. It is specially necessary to refer to this because 

 arrests have frequently been assumed upon the subjective grounds above 

 indicated without definite proof of them being existent. Thus, for 

 example, Celakovsky has lately traced all of the flowers of the 

 Gymnospermae from hermaphrodite flowers, and chiefly because in 

 Welwitschia a rudimentary ovule is present in the male flower. This is 

 a pure construction of the imagination, and the assumption that 'function- 

 less structures are always only the vestiges of former completely formed 

 ones which functioned as normal organs ' is no more, nor generally, 

 valid, as will be concluded from what I have said above, and before now 

 elsewhere, than would be the assumption that the ancestors of men 

 were hermaphrodite because man possesses rudimentary mammae and 



1 Gocbel, Beitr. zur Entwicklungsgeschichte einiger Inflorescenzen, in Pringsh. Jahrb. xiv. 



