438 SYSTEMATIC BOTANY. 



The colours vary in this Order; they are red, purple, brown, olive, &c., 

 but never pure green, like the Confervoids. 



The Olive-coloured Seaweeds, including the Fucaceae (of which the 

 " Bladder-wrack " is the commonest example), and others of very different 

 organization, the Phaeosporeae and Dictyotaceae, exhibit a similar grada- 

 tion of form in the thallus. The lower forms of Phaeosporeae present 

 tufts of branched filaments ; the higher forms of these, and the Fucaceae, 

 have thick leaf -like or stem-like fronds of firm texture and sometimes 

 enormous dimensions ; many of them have a shrubby habit of growth, 

 and attach themselves to stones &c. by discoid or branched expansions 

 from the base, resembling superficially the roots of the higher plants, but 

 having no similar function or anatomical character. The thallus of the 

 larger forms is highly developed as to its tissues, having a distinct cortical 

 layer ; but the structure is strictly cellular, without a trace of woody fibre 

 or vascular elements. The colour is here usually olive, brown, or some 

 dull tint of green-brown ; never bright green, as in the Confervoids. 



Vegetative reproduction assumes a very important place in the multi- 

 plication of all Thallophyta. Throughout the Algae it is a constant phe- 

 nomenon, and one illustrating very beautifully the physiological homoge- 

 neity of the thallus. The lowest forms multiply by dividing into a number 

 of cells or segments which grow up to the dimensions of the parent ; this 

 occurs as the ordinary mode of growth, here confounded with reproduction, 

 in the Palmdlece (figs. 504, 512, B, , 6), Desmidiea, Diatomacea (fig. 503), 

 &c. But another still more remarkable form of vegetative reproduction 

 extends from these up to the highest Algae, namely the reproduction by 

 zoospores. This consists in the conversion of the semifluid contents of in- 

 dividual cells (the endochrome) into distinct corpuscles, and the expulsion 

 of these from the thallus by the bursting of the parent cell-membrane 

 (fig. 506, 509, 512, 0, d) ; these corpuscles are filled with green or olive- 

 coloured matter, except at one end, which is provided with cilia and is 

 sometimes spoken of as the " rostrum." The cilia are excessively minute, 

 and vary in number in different genera ; sometimes the whole surface of 

 the zoospore is covered by them. These zoospores are usually emitted at 

 a fixed hour in the morning, as the result of an endosmotic action which 

 causes the cell-wall to burst and set free the zoospores. Sometimes before 

 their liberation they are seen to congregate in one portion of the cell, and, 

 as it were, to strike against the cell-wall and cause its rupture. Their 

 activity seems to be directly dependent on the influence of light. After 

 moving spontaneously for some time, the zoospores lose their cilia, become 

 encysted, and grow up into new thalli. In Botrydium and Acetabularia 

 conjugation of the zoospores has been recently observed. In some Algae, 

 moreover, there are two sorts of zoospores large ones, called macrozoo- 

 spores, and smaller ones, called microzoospores. Pringsheim even describes 

 a form of zoospore which has the faculty of remaining dormant for long 

 periods, and even of resisting desiccation for several months, at the expira- 

 tion of which time, if circumstances be favourable, it germinates and forms 

 a new plant. To such zoospores the name of chronizoospore has been given. 

 The formation of zoospores may take place in any or all of the cells of the 

 thallus of the filamentous and foliaceous Confervoids ; it occurs in certain 

 definite parts of the thallus of the Phaeosporeae, where there is a difference 

 in the constituent tissues. It has not been observed in the Red Seaweeds 

 or the Dictyotaceae where, however, a distinct kind of organ is found, 



