134 A TEXTBOOK OF THEORETICAL BOTANY 



enlargement into which the contents of the zoospore migrate. The nucleus 

 in the zoospore divides, and one daughter nucleus passes into the enlargement 

 while the other degenerates. The cell formed by the enlargement divides 

 several times, and the mature gametophyte consists of a short filament 

 of cells with shorter branches. It may even be reduced to two or three cells. 

 The male gametophyte is composed of smaller cells than the female 

 gametophyte (Fig. 122). 



'i'he sex organs arise from the lateral branches. The antheridium is a 

 small, more or less spherical, cell which gives rise to one antherozoid. This 

 anthe'rozoid is a small, oval cell provided with a pair of fairly long, unequal, 

 laterally placed flagella. The oogonium is also formed from a cell of a 

 lateral branch, inside which is a single oosphere. The oogonium is 

 surrounded by a thick mucilaginous investment which is prolonged at the 

 apex into a cup comparable with an egg cup. The oosphere emerges from 

 the oogonium and lies in the cup with the greater part of the oosphere exposed. 

 Fertilization is efTected in the usual way by the migration of the anthero- 

 zoid to the oosphere and the union of the male and female nuclei, after which 

 a wall is formed around the oosphere, resulting in the formation of an oospore. 

 Male and female gametophytes are found in equal numbers, and it has been 

 shown experimentally that from any zoosporangium an average of sixteen 

 male and sixteen female gametophytes will be produced. It follows that 

 normally each oosphere should become fertilized, and this takes place quite 

 rapidly. Should the oosphere fail to receive a male gamete it may be capable 

 of independent development producing a parthenogenetic sporophyte. 



The development of the oospore begins with its division into two cells, 

 from which is produced a filament with an apical cell. At first the young 

 plant remains attached to the top of the oogonium, but it later becomes 

 detached and one or more of the basal cells of the filament elongate into 

 unicellular rhizoids which form the primary attachment to the substratum. 

 Next the upper part widens into a monostromatic blade on a filamentous stipe. 

 The blade then becomes distromatic and the stipe becomes polysiphonous. 

 A new meristematic region appears between stipe and blade ; apical growth 

 gradually ceases and the apex of the frond is eroded. The first appearance 

 of the cortex is as a single layer of cells between the two layers of the blade. 

 The cells are large and parenchymatous, and are increased by the addition 

 of new cells in the meristematic zone. The medullary hyphae appear as the 

 cortex becomes double-layered, and the expansion of the trumpet hyphae 

 takes place in the intercellular spaces. 



Alternation of Generations 



Laminaria thus exhibits an alternation of generations comparable with 

 that in Dictyota, but whereas in Dictyota the two generations are morpho- 

 logically alike, in Laminaria the gametophyte is reduced to a minimum, 

 producing little more than the essential sex organs, while, on the other hand, 

 the sporophytic generation shows the greatest tissue elaboration found in the 



