280 BOTANY 



organs are no longer formed, although the young plants arise, by a vegetative process 

 of budding, from exactly the same part of the prothallium where the arehegoma 

 would have been developed. In the ease of Aspidium filix mas var. cristatum the 

 apogamy seems to have resulted from cultivation. In a broad sense the develop- 

 ment of bulbils in the place of flowers, in the species of Allium, might be con- 

 sidered as an example of apogamy. 



Parthenogenesis, or the development of an egg-cell without previous fertilisa- 

 tion, might also be viewed as an instance of the same phenomenon in plants with more 

 advanced sexual differentiation. In only one case, Ohara erinita, has parthenogenesis 

 been positively proven. The female plants of this species of Ohara are widely 

 distributed throughout Northern Europe, and develop normal plants from their 

 egg-cells, although the male plants are found only in Asia and in South Europe, 

 so that fertilisation could not have taken place. The egg -cell of Ohara erinita 

 has thus lost its special sexual character without altering its external appear- 

 ance. The essential sexual attribute of being incapable of further development, 

 without fusion with a male cell, has disappeared ; it has become a vegetative cell. 



Vegetative Multiplication by Single Cells (Spore - Formation). 



— As in the case of multicellular vegetative bodies, multiplication can 

 be effected also through the separation of single cells. Strictly speak- 

 ing, this manner of multiplication actually takes place whenever a 

 division of the vegetative body occurs in unicellular Bacteria, Fungi, 

 and Algae. Cells which serve the purpose of vegetative reproduc- 

 tion, and have a special form and method of development, are first met 

 with in the higher Cryptogams. They are frequently formed in special 

 organs or receptacles. Such organs, in the case of the Fungi, are the 

 sporangia or conidiophores, and the more complicated fructifications in 

 or on which the spores are formed. Instead of spores with cell walls 

 many Algae develop swarm-spores, which propel themselves in the 

 water by means of cilia, and are thus enabled to seek out positions 

 favourable for germination (ef. p. 243). In all higher Cryptogams 

 (Mosses, Ferns, Equisetaceae, etc.) the vegetative reproductive cells 

 are produced in peculiar multicellular sporangia, which open spontane- 

 ously by hygroscopic movements when the spores have reached maturity. 

 Among the higher Cryptogams there is not developed from the 

 spore a daughter plant similar to the parent, but there results an 

 entirely differently organised structure, which, by sexual reproduction, 

 produces a plant bearing spores, and similar to the original form. 



Sexual Reproduction 



For the purpose of sexual reproduction two kinds of cells, male 

 and female, are produced. Although neither alone is capable of develop- 

 ment, the actual reproductive body is formed by the fusion into one 

 cell of two such sexually differentiated cells. It has already been 

 pointed out that through such a union of two distinct cells, qualitative 

 changes may arise in the resulting organism, which would not have 

 been possible had it been produced by merely vegetative processes. 



