COLONIES OR CO EN OB I A IN THALLOPHYTA 31 



of the colony all behave at first in like manner; all can grow out into branches, 

 and thus perfectly irregular branching may take place (Fig. 6, ///). But there is 

 a difference between the branches and the chief stem it is only the energids of 

 the branches which can become propagative cells 1 . The chief stem apparently 

 serves only as a mechanical support for the whole, and it dies off at a later 

 period. The capacity for development of the energids at its base has evidently 

 become modified, because the claims made upon the chief stem as a mechanical 

 support are quite different from those laid upon the lateral branches living in 

 quickly flowing water the energids have changed their nature through the changed 

 demands made upon them. The growth of the branches is also remarkable. At 

 the apex the gelatinous layer is thinner 

 and an energid is found here which, 

 according to Klebs, divides by a longi- 

 tudinal wall ; after this it is displaced 

 and another energid occupies the apex. 

 Whilst then the single energids are like 

 one another in all essentials, the apex 

 of the colony always has a different 

 structure from the part lying behind it. 

 This is at least an approach to the 

 formation of a vegetative point. Pro- 

 bably also the threads of many fila- 

 mentous Algae such, for example, as 

 the Oedogoniaceae, Confervaceae, and 

 others, are nothing else than colonies 

 of swarm-spores which for a time are 

 invested with a membrane. The basal 



cells of these threads, which grow out 

 to fixing organs, behave usually like 

 the basal portion of the chief stem of 

 Hydrurus, that is to say, they are 

 deprived of their capacity for develop- 

 ment. In Ulothrix zonata, for example, 

 the protoplasm in the basal cell fre- 

 quently dies and thus divisions altogether cease in it whilst the other cells all go 

 on dividing as before (Fig. 7). The reason for this phenomenon may be the same 

 as that which we have above attempted to establish for Hydrurus. 



FIG. 6. Hydrurus foetidus. I young plant. // apex 

 of a plant ; chromatophore of the end-cell already 

 divided. /// branching. / and // after Klebs. /// after 

 Berthold. 



The examples with which we have just dealt show us that, starting 

 from colonies of similar cells or energids all of which are capable of 

 further development, an attempt is made in different directions to secure 

 a higher differentiation as this is expressed in the division of labour 



1 See G. Klebs, Flagellatensluclien, in Zeitschr. f. wissensch. Zool. Iv (1892), pp. 265, 353. The 

 literature is ?iven here. 



