PELAGIC PLANT LIFE 



usually almost globular development-stages that live in symbiosis 

 with various animals, and, in particular, with radiolaria. Of 

 these radiolaria, which would seem from Brandt's investigations Brandt. 

 to derive special benefit from the assimilation-products of algae, 

 we occasionally get the colony-forming species and Acantho- 

 metridae in such myriads among the surface-layers, that they 

 contribute a very large proportion of the organic substance 

 produced. I have previously stated that the brown algae also 

 regularly associate with a whole series of Dinophysidae. Another 

 family of brown flagellates includes the species of Phceocystis, 

 which form large colonies visible to the naked eye, and enveloped 

 in a loose slime (see Fig. 240). In cold waters these have actually 



been known to occur in sufficient 

 numbers to stop up the mieshes of 

 silk nets, and render them ineffec- 

 tive in working.^ 



It is the brown algae that, 

 properly speaking, characterise the 

 plant-world of the sea. Still there 

 are two other important series, the 

 cyanophycese and the chloro- 

 phyceae, which preponderate in 

 fresh water, and are, no doubt, re- 

 presented in salt water also, though 

 by only a few species. 



The Cyanophyceae are chiefly Cyanophyce^. 

 to be met with in warmer seas, if 

 we except the brackish water forms 

 that may be found along the coasts 

 of North Europe in the height of the summer. The genus 

 T7Hchodesmmm appears as clusters of threads, composed of Tnchodes- 

 brownish-yellow or red cells, which are either parallel to one """'"' 

 another, or twisted together, or matted and tangled, and 

 radiating in all directions. Wille, who described these forms wnie. 

 collected by the German Plankton Expedition in 1889, showed 

 that all the types may belong to the same species, Tricho- 

 desinium thiebaulti, under different development-forms. The 

 clusters may be seen sometimes when they collect near the 

 surface in calm weather, and resemble yellowish-brown snow- 

 flakes. Like the different kinds of fresh-water forms, they can 

 raise themselves in the water by means of vacuoles that, accord- 

 ing to Klebahn, contain air. When abundant they sometimes Kiebahn 



^ See Summary of Results Chall. Exp., p. 499, 1895. 



Fig. 2^0.— PiiyEOCYSTis povcheti. 

 (Lagerheim.) 



