570 Transactions of the Society. 



hioldia sanguinea is favourable for this experiment. The cell-wall 

 in young leaf-like portions of the thallus being thin, while the older 

 axial parts have the walls much thickened and exhibiting very 

 clearly lines of strati fi cation and striation. An additional proof in 

 favour of the thickening being due to apposition, is met with in 

 Ceramium riibrum, Polysiphonia fastigiata, and other species of 

 the same genus, where the axial cell, after a certain amount of 

 surface growth, is cylindrical with flat ends, the diameter of the 

 posterior end being often slightly greater than that of the body of 

 the cell, owing to a slight contraction of the cylindrical outer wall. 

 This contraction leaves a little channel inside the base of the cell, 

 and portions of the protoplasm which occupy this channel are cut 

 off from the rest by the thickening matter subsequently deposited, 

 which does not in all places follow the indenture of the wall. 

 These isolated portions of protoplasm, by subsequent growth, burst 

 through between the pericentral cells and form irregular cortical 

 cells on the surface of the stem. In most seaweeds portions of 

 the thickened cell-walls, more especially in the younger parts, 

 become resolved into mucilage, which in species with a fleshy 

 thallus, cements cells together that were otherwise free from each 

 other, so that a transverse section presents the appearance of com- 

 pact cellular tissue. It is due to the presence of this mucilage that 

 most algae adhere so firmly to paper when dried. 



In the simpler green seaweeds belonging to Harvey's Chloro- 

 spermese, illustrated by such genera as Pleurococcus and Glceocapsa, 

 we have probably the prototypes of existing vegetation. In such 

 the mode of reproduction is vegetative, and effected by fission, the 

 entire mass of the individual, after reaching a given stage, break- 

 ing up into a definite number of pieces, frequently four, each 

 capable of assimilating food until it reaches the size of its parent, 

 when fission is repeated. This mode of reproduction is also 

 characteristic of the lowest forms of animal life. It is interesting 

 to note, that in those organisms where reproduction is effected by 

 fission, there is no provision for death, as generally understood. A 

 Pleurococcus after having performed all the chemical and physical 

 functions necessary for the perfect developement of the species, 

 loses its individuality when fission takes place, but all the material 

 appropriated by life is retained, each succeeding generation reducing 

 the limited supply of available food capable of being converted into 

 its own substance. The false start made by the newly-evolved 

 force life, which would — if this primary idea of vitalizing and re- 

 taining in that condition all available material had been adhered to 

 — have resulted in its own extermination on the exhaustion of the 

 already existing supply of food, was corrected, and the continuance 

 of life for an indefinite period secured, so far as depends on the 

 presence of an inexhaustible supply of food, by the evolution of a 



