622 THE SUBDIVISIONS OF THE VEGETABLE KINGDOM. 



that systematic observations are being made of the organisms which occur at the 

 surface (e.g. at the Biological station on the Ploner See, Schleswig-Holstein) we may 

 hope that these lacunae in our knowledge may be filled up. Recent investigations 

 (by Klebahn) upon several of these "flowers of the sea" (Gloiotrichia echinulata, 

 Anabcena Flos-aquce, Aphanizomenon Flos-aquce, Trichodesmium, &c.) seem to 

 indicate that they possess special organs of flotation designated " gas-vacuoles ". 

 It would appear that these natant forms have in consequence a smaller specific 

 gravity than the surrounding water, and if the surface be quite unruffled tend 

 to float, whilst any disturbance, such as waves, &c., is sufficient to cause their 

 distribution through the upper layers of the water. Whether these " flowers " pass 

 another stage deep down in the water is not fully ascertained. Their spores, so 

 far as they have been observed, do not seem to possess " gas-vacuoles ", and sink to 

 the bottom. The phenomenon here indicated is not unlike that occurring in the 

 Protozoon Arcella, the protoplasm of which is able by secreting a bubble of gas to 

 rise to the surface, and, by absorbing it, to cause the organism to sink. The 

 Oscillarias consist of filaments of disc-like cells ; they exhibit curious gliding 

 movements, which have been already alluded to (cf. vol. i. p. 40). Rivularia is 

 distinguished by the fact that its filaments are whip-like, ending in a fine point, 

 whilst in Scytonema this distinction of base and apex is not found. They generally 

 occur in more or less mucilaginous masses. 



A number of the Schizophyceae are associated with certain Fungi to form 

 Lichens (cf. later, and vol. i. p. 244); and certain of them occur embedded in the 

 tissues of other plants. Thus species of Nostoc are met with in certain Liverworts 

 (Anthoceros) and in the roots of Cycas; and Anabcena in special cavities in the 

 leaves of the Rhizocarp Azolla. It is not known what may be the exact physio- 

 logical relations between these Blue-green Algae and the plants they inhabit 

 whether they are parasitic or symbiotic. 



Alliance III. Sehizomycetes, the Bacteria. 



On the whole, in the matter of their structure and aggregation, present many 

 characters in common with the Blue-green Algae. They are, however, destitute of 

 the characteristic pigment of that group, and pass their lives as parasites and 

 saprophytes, obtaining their food from ready-formed organic matter. Nuclei have 

 been distinguished in some few forms, and the cells are inclosed in a membrane 

 which, though often mucilaginous, does not consist of cellulose. In size the cells 

 reach very small dimensions, and may be regarded as the smallest of all plant- 

 forms. A number of different forms are distinguished; the Cocci, consisting of 

 minute round cells; short rod-like forms, the Bacteria; longer rod-like forms, the 

 Bacilli; various spiral forms known as Spirillum, Vibrio, and Spirochaete; filamen- 

 tous forms, Leptothrix and Crenothrix. A very common mode of occurrence of the 

 various forms is in gelatinous masses, to which the name Zoogloea has been applied 

 (cf. fig. 368 2 ). Propagation takes place vegetatively by cell-division so long as the 



