56 



EVOLUTION OF PLANTS 



third order, the Characese, is made up of very peculiar 

 plants of doubtful affinities. 



The SiPHONEiE 



This order contains a good many types differing a 

 good deal among themselves and showing in some cases 



a high degree of special- 

 ization. They differ 

 from the other green 

 forms in the almost 

 complete absence of 

 division vsralls within 

 the plant body, although 

 they can hardly with 

 propriety be considered 

 as strictly unicellular 

 since the protoplasm 

 contains a large number 

 of nuclei. The plant 

 may be a simple tubular 

 filament, or it may be extensively branched and form a 

 body of considerable size showing a remarkable degree 

 of external differentiation, actually mimicking the struct- 

 ure of the higher plants in the development of stem, 

 leaf, and root (Fig. 11) ; but even in such cases the 

 hollow cavity of the thallus is undivided by partition 

 walls. The wall is lined with a protoplasmic layer in 

 which are imbedded the numerous nuclei and chloro- 

 plasts. The division of the nuclei, of course, is not 

 accompanied, as in most cells, by the formation of a 

 division wall. 



Fio. 11.— Part oi a plant of Caulerpa 

 plumaris, one of the Siphonese, show- 

 ing external differentiation into stem, 

 root, and leaf in a non-cellular plant ; 

 X, the growing point ; r, rootlets. 



