22 FIRST GROUP. — THALLOPHYTES. 



are able to pass through a period of rest; or in the species which form cell-filaments, 

 such as the Nostocaceae, by hormogonia, — portions of filaments with power of 

 motion, which become isolated, and after a period of rest grow into new individuals. 

 I have besides seen the formation of swarm-spores in Merismopedia. 



The Cyanophyceae fall into two divisions ; in one of these, the Chroococcaceae, the 

 cells lie, singly or a number together variously arranged, in a gelatinous envelope 

 formed by the swelling up of the cell-walls ; in the second division, that of the Nosto- 

 caceae, the cells are united into filaments. 



1. The CHROOCOCCACEAE live as isolated roundish cells or in roundish families, 

 whose cells are imbedded in amorphous mucilage or in the swollen walls of their 

 mother-cells. They occur as gelatinous growths in damp 

 places. Several genera with numerous species have been 

 distinguished: Chroococcus and Gloeocnpsa (Fig. ii), which 

 divide in all directions, the latter imbedded in a stratified 

 jelly ; Gloeotheca in a similar jelly but dividing only in one 

 direction ; and Merismopedia, the cells of which divide cross- 

 wise and lie in a single plane. 



2. The NOSTOCACEAE. The genus Nostoc may be 

 taken as a typical example of this division. It forms lumps 

 of mucilage or gelatinous nodular envelopes, which float in 

 water or lie loose on damp earth or among mosses. Monili- 

 form rows of round cells lie coiled up like snakes in the 

 Gioeocafsn. ^^jj^ ^ sluglc larger cells, termed heterocysts, incapable 



of further development and having differently coloured watery cell-contents, are 

 interposed at intervals in the chains. The filaments increase in length by the growth 

 and transverse division of the individual cells, and thus add to the number of the coils 

 in the jelly which they secrete. New colonies are formed, according to Thuret, in the 

 following manner :— the jelly of the old colony becomes softened in the water, and the 

 portions of the filaments that lie between the heterocysts creep out of the jelly, while 

 the heterocysts themselves remain in it. Having come out into the water these 

 portions of filaments, termed hormogonia, exhibit the movements of the Oscillatorieae ; 

 their escape from the jelly is probably brought about by these movements as well as by 

 the liquefying of the jelly itself The hormogonia may continue in motion for as long 

 a time as one hour '. When they have come to rest they stretch themselves out and 

 surround themselves with a gelatinous envelope. The roundish cells of the hormogonia 

 now grow transversely, i, e. at right angles to the axis of the filament, and divide by 

 longitudinal walls parallel to its axis; but the short filaments thus produced continue to 

 be connected with one another at their extremities, and thus form the beginning of a 

 single coiled nostoc-thread. Individual cells, disposed apparently in no definite order, 

 become heterocysts. For the formation of spores certain cells— often all the cells of a 



' These motile threads of Nostoc were seen by Janczewski to enter the young stomata of the 

 under side of the thallus of Ajtthoceros laevis, where they develope into roundish coils. Such Nostoc 

 colonies have long been known in cavities and in the tissue of certain Hepaticae {Blasia and 

 An/hoceros), but they were usually regarded as endogenous gemmae of these species, till Janczewski 

 pointed out their true nature. N^osioc also forms settlements in the large porous cells of the leaves of 

 Sphagnum. The entrance of Nostoc into the parenchyma of the stem of a dicotyledonous plant 

 Gunnera is effected, according to Reinke, in a different way ; the deeper lying cells of the parenchyma 

 of the circumference of the stem, which are themselves covered by layers of parenchyma, are densely 

 filled with colonies of the Alga (Bot. Zeit. 1872, pp. 59 and 74). An Anahaena is found constantly 

 in the cavities of the leaves of Azolla (Salviniaceae) in the earliest stages of the growth of the young 

 plant; the filaments oi i}nG Anabaeua, according to Berggren, attach themselves to the macrospores, 

 and as soon as the embryo is formed they make their way into its leaves. 



