4 FIRST GROUP. 



cell-sap -nd other constituents are only imperfectly distinguishable. From this initial 

 stage the process of development may advance, yet still within the limits of a single 

 cell ; and while the cell increases in size, often reaching dimensions without parallel 

 in the vegetable kingdom, either the differentiation of the cell-contents, or that of the 

 external form as shown by the branching, may make most rapid progress. In other 

 cases the growth of the cells is accompanied by cell-division, the thallus becoming 

 multicellular, and the single cell producing, according to the nature of the plant, a 

 cell-row or a cellular filament, a cell-surface or simple tissue-layer, or lastly a cell- 

 mass increasing in size in every direction. 



Each of these processes is subject to a number of variations. We find for 

 instance cell-rows, in which the connection of the individual cells is but superficial ; 

 they consist in fact of unicellular Thallophytes which are merely strung together, and 

 the cells may readily part from one another and commence a separate existence. 

 Other cell-rows again show a differentiation into base and apex ; in this case the cells 

 which form the base are usually developed as organs of attachment, rhizoids (roots). 

 Again, larger or smaller circumscribed masses of tissue may be formed by the union 

 of cell-rows that were originally free ; in some cases, as in many Fungi, massive 

 bodies arise from the simple interweaving of cell-rows. On the other hand, the 

 vegetative body of the Myxomycetes consists of membraneless naked masses of pro- 

 toplasm endowed with the power of independent motion. 



There are simple Thallophytes also, which show a tendency to pass a longer 

 or shorter portion of their existence in the condition of freely motile membraneless 

 primordial cells (swarm-spores for example) ; in this form they are more or less 

 like the simpler Infusoria, and were till quite recently confounded with them. Cases 

 also occur in which cells that have already clothed themselves with cellulose, and 

 even assemblages of many such cells, swim freely about in water and for a con- 

 siderable time. But the motile condition is in all cases interrupted by longer 

 stationary periods, during which growth and increase in volume usually take place. 

 In the case of some of the more highly developed Thallophytes the motile con- 

 dition is confined to the spermatozoids, the male elements in fertilisation, and in 

 many even this form of independent motion is wanting, as for example in the Florideae. 



The variety in the modes of reproduction in the Thallophytes is as great as 

 that of the structure of the vegetative body. We find at first the very simplest 

 forms, and we arrive at last at modes of reproduction almost as perfect and as 

 complex as are to be found even in the highest plants. In the simpler forms the 

 unicellular or pluricellular thallus exhibits two modes of increase ; it breaks up 

 after a period of growth into separate pieces, each of which continues to live and 

 grow independently, as happens with the Schizomycetes and Cyanophyceae ; or 

 separate cells of the thallus persist after the decay of the others and become 

 resting cells, being endowed with more than usual power of withstanding influences 

 from without, especially the effect of desiccation. But both asexual and sexual 

 reproduction are found in the majority of Thallophytes, and phenomena occur in 

 the higher forms which are comparable with the alternation of generations in the 

 Vascular Cryptogams. The organ of reproduction which separates from the 

 mother-plant is almost always a single cell, but the origin, significance and power 

 ,of development of this cell may vary much. 



