io8 CHLOROPHYCEAE 



Spirogyra. Lloyd, F. E. (1926). Trans. Roy. Soc. Can. 3rd series, 20, 75. 



Spirogyra. Lloyd, F. E. (1928). Protoplasma, 4, 45. 



General. West, G. S. (1915). jf. Bot. 53, 73. 



Zygogonium. West, G. S. and Starkey, C. 6.(1915). New PhytoL 14, 194. 



*CHARALES 



The plants forming this small order represent a very highly 

 specialized group that must have diverged very early in the course 

 of evolution from the rest of the green algae, the intermediate forms 

 subsequently being lost. They are characterized in that they lack 

 asexual reproduction and possess very complex sexual reproductive 

 organs. The young plants develop from a protonemal stage, the 

 erect plants having a structure which is more elaborate than any 

 type so far described, whilst the thallus is also frequently lime 

 encrusted. The group is very ancient because fossil members are 

 found from almost the earliest strata. The living forms are widely 

 distributed in quiet waters, fresh or saline, where they may 

 descend to considerable depths so long as the bottom is either 

 sandy or muddy. 



*Nitella {nitella, a little star). Figs. J 4.-77. 



The plants have the appearance of miniature horsetails (Equise- 

 tum) because they bear whorls of lateral branches arising from the 

 nodes. The nodes are formed by a transverse layer of cells in 

 contradistinction to the internodes, which consist of one large cell 

 whose individual length may extend up to 25 cm. in Nitella cernua. 

 The height of the different species varies up to i m., growth being 

 brought about by an apical cell which cuts off successive segments 

 parallel to the base. Each new segment divides transversely into 

 two halves, the upper developing into a node and the lower into an 

 internode (fig. 75 B). Branches, both primary and secondary, are 

 formed by the peripheral cells of the nodes protruding to form new 

 apical cells, but these soon cease to grow after the branch has 

 reached a short length. At the basal node of the main plant 

 branches of unlimited growth are produced: these arise on the 

 inner side of the oldest lateral in the whorl, thus producing a 

 fictitious appearance of axillary branching. Multicellular branched 

 rhizoids with oblique septa function as absorption organs and also 

 serve for anchorage. The rhizoids develop from the lowest node of 

 the main axis, but every node is potentially capable of producing 



