BOTANY 



Fig. 68.— Free cell forma- 

 tion in the fertilised 

 egg - cell of Ephedra 

 altissivw. (X 100.) 



(Fig. 68) ; but the four or eight cells thus formed have no contact, 

 with each other, and the cytoplasm of the mother cell is not totally 

 consumed by their formation. 



Cell-Budding.— This is simply a special variety 

 of ordinary cell division, in which the cell is not 

 divided in the middle, but, instead, pushes out a 

 protuberance which, by constriction, becomes separ- 

 ated from the mother cell. This mode of cell 

 multiplication is characteristic of the Yeast plant 

 (Fig. 2, p. 11) ; and the spores, known as conidia, 

 which are produced by numerous Fungi, have a 

 similar origin (Fig. 286). 



Cell-Formation by Conjugation. — A sexual 

 cell is only able to continue its development after 

 fusion with another sexual cell. The two cells so 

 uniting are either alike, and in that case are called 

 GAMETES, or unlike, and are then distinguished as 

 egg and spermatozoid. The spermatozoid is the 

 male, the egg the female sexual cell. The gametes 

 may be motile or non-motile (Fig. 69, B). The 

 motile gametes frequently resemble the swarm- 

 spores (Fig. 69, A) generated by the same parent for the purpose 

 of asexual reproduction. As a rule, however, they are smaller 

 than the swarm-spores, and have usually only half as many cilia. 

 In the more highly specialised sexual cells the egg usually retains 

 the structure of an embryonic cell, but the spermatozoid under- 

 goes various changes. A cytoplasmic cell body, a nucleus, and the 

 rudiments or chromatophores are always present in the egg. The 

 male sexual cell (Fig. 70), on the other hand, becomes transformed, 

 in the more extreme cases, into a spirally twisted body, provided with 

 cilia, and exhibiting an apparently homogeneous structure. Only a 

 knowledge of the history of its development, and the greatest care in 

 hardening and staining, have rendered it possible to recognise the 

 homology of the structure of such a spermatozoid with that of an 

 embryonic cell. It has been shown that one part of its spiral body 

 corresponds to the cell nucleus (k), another, together with the cilia, to 

 the cytoplasm (c), and the vesicle (6), at the other extremity, to the sap 

 cavity of a cej.1. After the spermatozoids are set free from the sexual 

 organs, they require water for their dispersal. They are motile, and 

 are thus enabled to seek out the egg-cells, which, in most cases, await 

 fertilisation within the organ in which they have been formed. 



Motile, male sexual cells occur only in the Cryptogams. In the 

 Phanerogams (Fig. 71) the non-motile male cell (gz) is carried to the 

 egg by the growth of the pollen tube (Fig. 71, A), in which it is 

 enclosed. In the union of the two sexual cells in the act of fertilisa- 

 tion, the egg nucleus (ek) and the sperm nucleus (sk) fuse and form 



