THE SIPHON ALG.E 197 



earths have so large a proportion of hard diatom shells as to 

 he valuable as polishing powders, and they are also used as the 

 absorbent of nitroglycerin in the manufacture of dynamite. 



ORDER 5. THE SIPHON ALG^E, OR SIPHONALES 



227. The siphon algae. The siphon algae (order SipJionales) 

 differ from all other groups of algse in the striking peculiarity 

 that the protoplasm, with thousands of nuclei, is not separated 

 into compartments or cells, but is all contained within a com- 

 mon filament or other cell cavity. Such a many-nucleate struc- 

 ture is called a ccenocyte (meaning a vessel in common). The 

 siphon alg are chiefly marine, and many large and complicated 

 forms are found in the warmer seas (Caulerpa, Udotea, etc.). 

 Some of these are heavily incrusted with lime (Acetabularia, 

 Penicillus, Halimeda, etc.). Two-ciliate zobspores and gametes 

 are developed by certain types in cells cut off from the ends of 

 the filaments. The gametes fuse in pairs on their escape into 

 the water, forming zygospores. All of the siphon algse are isoga- 

 mous, when sexual at all, except the green felt, Vaucheria, which 

 stands quite alone as the only heterogamous type in the group. 



228. Vaucheria. Vaucheria, the green felt, is a very common 

 alga, forming mats of coarse filaments on the muddy bottom -of 

 shallow pools and ditches. Some species are terrestrial and may 

 often be found as thread-like growths over the damp earth of 

 flowerpots in greenhouses. The filaments are long and sparingly 

 branched, and are, of course, continuous tubes without cross 

 w r alls except where reproductive organs are developed. Immense 

 numbers of small, disk-shaped chloroplasts (Fig. 189, F) are 

 present in the layer of protoplasm under the cell wall, and the 

 very small nuclei lie among them. The protoplasm contains 

 numerous globules of oil, which in this plant takes the place of 

 starch as the first visible product of photosynthesis. 



The zoospores of Vaucheria are very large, many-nucleate 

 and many-ciliate structures, visible to the naked eye. They are 



