278 BOTANY tart i 



the vegetative body occurs in unicellular Bacteria, Fungi, and Algae. 

 Cells which serve the purpose of vegetative reproduction, and have 

 a special form and method of development (spores, conidia, swarm- 

 spores, gemmae), are first met with in the higher Cryptogams. They 

 are frequently formed in special organs or receptacles (sporangia, 

 conidiophores, fruit bodies). 



B. Sexual Reproduction 



Sexual reproduction is also wide spread in the vegetable kingdom, 

 and is Avanting only in such lowly organised groups as the Bacteria 

 and Cyanophyceae and in some Algae and Fungi. 



In its typical form tAvo distinct cells, Avhich are distinguished as 

 the female or egg-cell and the male or sperm-cell, give rise after their 

 fusion to a new plant. Neither the egg-cell nor the sj^erm-cell can 

 be strictly regarded as a reproductive organ, since neither by itself is 

 capable of further development. Only when the egg-cell has been 

 fertilised, i.e. has united with the sperm-cell, is a structure which 

 subserves reproduction produced. 



Sexual reproduction is met with in the vegetable kingdom in very 

 difierent forms ; the extremes are very distinct but are connected by 

 many intermediate forms. Only a few can be mentioned here (cf. the 

 special part). In the lowest grade we find two completely similar 

 motile cells (gametes) ; these are evidently to be phjdogenetically 

 derived from swarm-spores, but are distinguished from them by 

 undergoing further development only after fusion (certain BroAvn and 

 Green Algae). At a slightly higher stage one of the two gametes is 

 distinguished by its size and by losing the power of movement earlier 

 than the other one; it is the female gamete, and when at rest is 

 sought out and fertilised by the male gamete. At a further stage 

 this egg-cell has completel}' lost the power of movement ; it remains 

 in its mother-cell the oogonium, where it is reached by the male cell 

 (spermatozoid), which enters by an opening in the wall of the 

 oogonium (Oedogonium, Chara). Essentially similar relations hold for 

 all Bryophyta and Pteridophyta, onlv in them instead of unicellular 

 oogonia we meet with archegonia. The archegonium is an organ 

 which forms the egg-cell enclosed within an investment of cells which 

 ultimately opens at a definite place. 



In all these cases fertilisation takes place in water. The egg- 

 cells excrete certain substances by means of which spermatozoids 

 are attracted. Among Thallophyta there are also forms in which 

 motile sperm-cells are wanting and the approximation of the sexual 

 cells is a result of growth. Either both cells grow eci[ually towards 

 one another (cf. e.g. Conjugatse, Zygomycetes in the special part) or 

 the male cell only grows to reach the female cell (cf. Saprolegnieae, 



