224 MORPHOLOGY OF MEMBERS. 



act is always that two cells of the simplest possible kind combine, either completely 

 coalescing with one another, or at least their contents becoming partially inter- 

 mingled so as to afford a starting-point for a fresh development. It is the 

 essential characteristic of sexual reproductive cells that each is incapable of 

 further development by itself, this being the result of the combined action of 

 two such cells. 



It is only in the lowest forms of vegetable life, in some Algae and Fungi, 

 that the two cells which take part in the act of sexual union are alike or at least 

 very similar in size, form, and physical properties. In this case their union is 

 called conjugation^ and the cell capable of germination which results from the 

 union a zygospore. In all other cases the two uniting cells are strikingly different 

 in size, form, and physical properties. In these cases one of the two cells, 

 the male cell, conveys to the other only a very small quantity of material by 

 means of which it produces an effect upon it ; this other cell, the female cell, 

 contains by far the largest proportion of the material which takes part in the 

 development incited by the act of union. With the exception of a few complicated 

 cases among Algae and Fungi, which will be particularly described, the relation- 

 ship between the two sexual reproductive cells is still more clearly indicated 

 by the fact that the male cell is motile, carrying to the other cell the fertilising 

 material. The motion of the male cell is however of two kinds ; it may either be 

 spontaneous, as in most Cryptogams, when the cell is termed an anther ozoid^ ; 

 or, as in Phanerogams, the male cell, then called a pollen-grain, becomes detached 

 from the parent-plant, and conveys the fertilising material to the female cell first 

 of all by the aid of external forces, then by its own growth. The female cell, on 

 the other hand, which is fertilised by means of the male cell, remains at rest at 

 the place where it was formed, or at most, as in the Fucaceae, is carried about 

 passively; with the exception of the cases above referred to it is always a naked 

 primordial cell, and is termed the odsphere, or germinal vesicle. After fertilisa- 

 tion, it secretes a cell- wall, and is then termed an oospore; from this, either at 

 once, or after a period of rest {resting spore), the young plant is subsequently 

 developed. Some not inconsiderable deviations from this plan will have to be 

 pointed out in the class Carposporeae of Thallophytes ; but even in these cases 

 the essential condition of fertilisation remains, that the male cell only incites to de- 

 velopment, while the development itself proceeds entirely from the female organ. 



Very considerable variety is shown in the morphological characters of the 

 sexual organs, if we take a comparative view of the whole vegetable kingdom ; 

 but in the larger groups of plants we find the morphology of these organs to agree 

 completely in all essential points, even when the anatomy of the vegetative organs, 

 the habit and mode of life of the plants that compose them, vary greatly. When 

 considering, in Book II, the distinguishing characteristics of the different classes 

 of plants, our attention will be specially directed to the morphology of the sexual 

 organs, and it will be sufficient here, as an introduction to what follows, to define 

 the most general terms connected with these organs. 



' [The term 'anthcrozoid' was first proposed by Derbes and Solier, Ann. des sc. nat. 1850, vol. 

 XIV. p. 263.] 



