32 GENERAL DIFFERENTIATION OF THE PLANT-BODY 



between the inhabitants of the colony. The term ' division of labour ' 

 of course explains nothing, it is merely a comprehensive expression for 

 the facts. The division of labour and the differences bound up with 

 it are brought about firstly by the reciprocal influence of the energids 

 of a colony and then by their different relations to external factors. 

 Energids are frequently, on account of their position in a colony, influenced 

 by other energids, for example, in Pediastrum, Polysphondylium, and 

 Volvox, and are thereby fixed in their form and function ; they are 

 further, on account of their position, differently affected by external 



FlG. 7. Ulothrix zonata. A germ-plant, 

 r anchoring organ which is poor in chloro- 

 phyll or has entirely lost its chlorophyll. 

 Magn. 300. The other figures do not concern 

 us here. After Dodel-Port. Lehrb. 



FIG. 8. Ulva lactuca. Germ-plant with 

 anchoring organsat base. Magn. 220. Lehrb. 



I-!,;. 7. 



factors, and react differently to external stimuli, as we have seen in 

 Hydrurus. 



An extensive division of labour can only be expected in branched 

 vegetative bodies. Branching occurs, as we have seen, even in colonies, 

 but it is regular only where a vegetative point exists, and the possession 

 of this is characteristic of cell- dominions and therefore of all 'typical' 

 plants. 



