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 Plöner 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, Aphanizoriienon Flos-aquce, Trichodesmium, kc.) 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 unrufiied 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 occuiTing 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). Bivularia 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 numljer of the Schizophyceee 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 Kliizocarp 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 HI. — Schizomycetes, 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-). Propagation takes place vegetatively by cell-division so long as the 



