566 



HUTCHINSON'S POPULAR BOTANY 



living plants, but take care to clean out all the protoplasm that they may 

 quarrel about the perforations and sculpturing of the valves, which appear 

 to vary according to the lens used for their definition. They are isolated 

 and free-swimming, or variously attached to each other or to higher 

 plants by gelatinous stalks. "Tripoli" and " Kieselguhr" are deposits of 



the fossil Diatoms of past epochs, which occur 

 in beds of considerable thickness in various 

 parts of the world. The number of species of 

 A Diatoms known is enormous certainly more 



than ten thousand ! In certain species of 

 Diatoms conjugation is effected by two indi- 

 viduals coming together, and after throwing 

 off their siliceous valves the protoplasts become 

 fused to form a new individual, which after- 

 wards multiplies by division. But in the 

 majority of Diatoms there is nothing ap- 

 proaching a sexual union, and the purely 

 vegetative multiplication by division appears 

 to be the rule in the Classes PERIDINE^E and 

 SCHIZOPHYTA. 



A few words should be said respecting the 

 Class Schizomycetes or Splitting Fungi, better 

 known as Bacteria, though they are so minute 

 that they cannot be studied, or even seen, 

 except by the aid of very high powers of the 

 microscope. In consequence there has been 

 much confusion as to form, structure, and 



$ development. They are single cells of spheri- 



cal, spiral, or cylindrical form, or combined 

 V^ to form chains and filaments. The cell-contents 



^\ is homogenous protoplasm, in a few cases 



tinged with chlorophyll. Some have the 

 power of free movement and of rotation round 

 the longitudinal axis; certain of these are 

 possessed of cilia or fiagella, but whether 

 these are their organs of motion has not 

 been made out satisfactorily. The spherical 

 forms are known as Coccus or Micrococcus, 



the cylindrical as Bacillus and Bacterium according to whether they 

 are relatively long or short, and the spiral forms as Spirillum. Coiled 

 forms other than spirals are known as Vibrio, and the compound fila- 

 ments as Leptotkrix and Beggiatoa. They are reproduced by the cell- 

 contents developing into spores, which are set free by the rupture of 

 the cell-wall, or in the filamentous species by the breaking off of cells 



FIQ. 715. YOKE-THREAD 

 (Zygnema). 



Two threads in proximity have united 



by tubes through which the cell-contents 



mingle to form zygosperms. 



