GROWTH AND ORIGIN OF MULTICELLULAR PLANTS. 268 
over external forces, which in the simpler Algw is reduced to the 
resistance offered by the external sheath. These conditions pre- 
dominate in the Chroococcacea, but are also met with in Phanerogams 
where cells are 0, from the pressure of surrounding tissues, as in 
some pollen-gra 
Differ aiitiation of the otter bd ~ Sea a : very early period 
of plant evolution, as in the Ose where Se 
except at one point, exerts a Genes initial nce on the extension of 
the protoplasm and Raps be 4 cell-formation, which is colitinnel 
in a straight line a from the starting-point, owing to the rapid 
movements are most pron ota species growing in shallow water 
or damp ee and conaeqaaialy ate to the. ‘cities of the 
atmos 
The cap — structure described by Borzi* as covering the 
wing-point in Oscillaria is in reality the relatively thick un- 
diff anaated porDh of the sheath, which contracts as it becomes 
cuticularised. The individuality of the sheath, and its importance 
in exercising a directive influence on growth is very clearly illus- 
rated by such genera as Microchete, Lyngbya, Rivularia, and 
Calothrixz, where, as in Oscillaria, the individual consists of an 
unbranched single row of superposed cells, enclosed in a fir 
Oran 
ante completion of the vegetative period, reproduction 
s by the formation of horm rmogonia ; in those species with 
heteroyat ah string of cells between two of the latter constitutes 
mogoni in the absence of heterocysts constrictions take 
sists at cater le: 4m portions becoming detached, and soon escape 
through the ruptured apex of the sheath into the surro ig 
water, st as first pointed out ae ar a Thuret,t+ they 
exhib 
of the sheath, which, according to the a authors, is secre 
within twenty-fo our hours in species of Lyngbya.t The number of 
cells forming a homogonium varies W1 with the species, from six to 
twenty or more, and, after the secretion of a sheath, becomes 
attached by one end and increases in length by cell-division at the 
apex. It is interesting to notice that the direction of the axis of 
growth of the parent plant is retained in the development of indi- 
viduals from hormogonia, which is not in harmony with the state- 
* Loc. cit. + ‘Notes Algologiques,’ fase. i. p. vi. tL. ¢. p. 4. 
