I §2 A TEXTBOOK OF THEORETICAL BOTANY 



domonas, Spirogyra, Fucus and Batrachospermiim. A life history of 

 the second category which includes two vegetative plants, whether 

 they are alike or not, is called diplobiontic. Examples of this are 



Cladophora, Coleochaete, Dicty- 

 ota, Laminaria, Cutleria and 

 Polysiphonia (Fig. 175). 



In a diplobiontic life-history, 

 if the two plants are morpho- 

 logically identical, the alterna- 

 tion between them is said to be 

 homomorphic. This applies 

 to Cladophora, Dktyota and 

 Jeuaspor- Polysiphotiia in the above list. 

 ong/um jj^ ^j^g opposite case, where 



the plants are dissimilar, the 



alternation is called hetero- 



morphic. This applies to 



Coleochaete, Laminaria and 



Cutleria among those mentioned 



above. 



The nse of these terms is 



quite independent of the cyto- 



logical life-cycle, and it matters 

 Fig. iy4.~Corallwa officinalis. Longitudinal section wbpfbpr the nlants ron- 



of a tetrasporangial conceptacle with tetraspores. ^^^ wnetner tne plants COn 



cerned are monoploid or diploid. 

 For example, as we have seen above, the life-histories of both Batracho- 

 spermum and Fucus are haplobiontic, but the vegetative phase of the former 

 is monoploid, while that of the latter is diploid. On the other hand in a 

 diplobiontic life-cycle it is inevitable that one vegetative plant of the cycle 

 must be monoploid while the other must be diploid. 



It follows therefore that, in considering the type of alternation of 

 generations exhibited by an Alga, the criterion is not whether the plant is 

 monoploid or diploid but whether one or two separate plants are necessary 

 to complete the life-cycle (Fig. 175). 



A plant in which the vegetative phase is monoploid is termed a monoplont 

 or haplont, while one in which the vegetative phase is diploid is termed a 

 diplont. Further, if there are two vegetative phases, whether similar or 

 dissimilar, the species is called haplo-diplont. These terms refer solely 

 to the cytological condition and are not dependent upon the type of alternation 

 of generations exhibited by the species. 



The Rhodophyceae present a further and peculiar condition, brought 

 about by the interpolation of a special post-fertilization carposporophyte 

 tissue which produces the carpospores. This tissue may be quite small 

 or it may be extensive, but it is always produced in organic connection with 

 the gametophyte. In forms like Batrachospermmn the carposporophyte is 

 the only diploid structure, and thus we have an alternation of two generations, 



