MetaphytaFucus. 89 



thallus performs for itself the important functions requisite 

 for the maintenance of life. 



Amongst the cells forming the thallus, however, we have 

 a certain amount of modification of form. As already seen, 

 the cells of the centre, or medulla, are different from those 

 nearer the surface, or cortex ; whilst those associated with 

 the reproductive sacs are still more variable in shape. 

 Collections of more or less similar cells are called tissues. 

 Thus we may speak of medullary tissue and cortical 

 tissue, or we may describe in general term the whole thallus 

 as being made up of a cellular tissue. Organs, therefore, 

 are made up of tissues, which are themselves collections of 

 more or less similar cells. 



It is important that we should pause at this point and 

 endeavour to ascertain what is the rationale of the pheno- 

 mena of true sexual reproduction which we have, in Fucus, 

 met with for the first time. 



In the first place, what is the connection, if any, between 

 asexual and sexual reproduction ? It was pointed out 

 (p. 52) that asexual reproduction meant the separation of any 

 cell from the parent, which, without union with any other 

 cell, was capable of developing into an organism like the 

 parent. A modification of this process is exhibited among 

 the higher plants, and more rarely among the higher animals ; 

 namely, the separation of many cells in the form of a ' bud,' 

 * shoot,' * stolon, 7 etc., which is similarly capable of deve- 

 loping into an adult organism. 



The process of protoplasmic anabolism (p. 36) was seen 

 to be one usually accompanied by growth in the early stages 

 of a plant's or animal's life-history. The various pheno- 

 mena of life are, of course, always possible only if the pro- 

 ducts of katabolism be got rid of. Now since the mass of 

 any growing cell increases as the cube of the dimensions, 

 while the surface only increases as the square (H. Spencer), 

 katabolism must sooner or later overtake and outstrip 

 anabolism, with one of two results to the cell, namely, 



