CELL-DOMINIONS IN THALLOPHYTA 33 



II. CELL-DOMINIONS WITH VEGETATIVE POINTS. 



We owe the special term vegetative point to the founder of the 

 history of development, Kaspar Friedrich Wolff 1 , who endeavoured to 

 establish by direct observation the origin of organs their generatio- 

 instead of accepting the speculations of the theory of evolution prevalent 

 at his time. Through this method he came to the conclusion that in 

 development an actual new formation of parts occurs, a new laying 

 down of organs on the originally undifferentiated germ. To this result 

 he was led by his investigation of the development of the leaf, as well 

 as of the flower, in the bean. He recognized that the existence of 

 primordia of leaves in the bud, upon which the doctrine of evolution 

 was based, afforded only a narrowly limited foundation. If you examine 

 accurately a bud ' donee tandem hoc modo introrsum et deorsum simul 

 penetrando ad substantiam plantae interiorem pervenias, humidam, succis 

 gravidam et nulla amplius folia tenentem ' and you will arrive at ' extremitas 

 axeos trunci ' where no differentiation of tissue yet exists. He calls 

 this termination of the stem-axis or twig-axis the vegetative point, and 

 upon it arise the primordia of leaves and lateral branches as ' propulsiones 

 trunci.' In this way one of the fundamental facts in the development 

 of plants was clearly established, namely, the plant-body possesses places 

 where, to use the expression of Sachs, 'embryonal' tissue still exists 

 which furnishes new cells and new organs. This is a feature which 

 distinguishes them at once from the higher animals. New organs usually 

 arise in such a way that the youngest are nearest the vegetative point ; 

 they are in progressive serial succession. A study of what may be 

 observed in the lower plants shows that there is little constancy in 

 this respect, and also that the possession of a vegetative point different 

 from the other parts is only a special case, although at the same time 

 the most widely spread one, of the possible constructions of the plant- 

 body. We have already seen in Hydrurus an approach to a vegetative 

 point ; a real vegetative point however requires that the cells composing 

 it should behave differently from those which are found behind it, and 

 that the primordia of the lateral formations should develop from the 

 vegetative point in a definite and regular succession. 



Amongst the species of Cladophora, one of the most widely spread 

 genera of the branched pluricellular filamentous Algae, we find a con- 

 siderable advance in this respect. The individual cells are here polyergic, 

 but this is of no consequence for our present purpose. In Cladophora 

 fracta 2 cell-multiplication takes place by the division of a cell occupying 



1 K. F. Wolff, Theoria generationis, 1758. 



2 See Berthold, Untersuchnngen iiber die Verzweignng von Siisswasseralgen , in Nova Acta Acad. 

 Leop.-Carol. xl (1878). 



C.OEBEI. D 



