304 BOTANY PAKT I 



ment. JICKELI regards the common feature of the various causes 

 which lead to parthenogenesis to be that they are injurious influences, 

 the effect of which, however, must not be carried too far ( 108 ). 



Vegetative Multiplication by Single Cells (Spores). As iu the case of multi- 

 cellular vegetative bodies, multiplication can be effected also through the .separa- 

 tion of single cells. Strictly .speaking, 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 (spores, fouidia, swarm- 

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

 formed in special organs or receptacles (sporangia, conidiophores, fruit bodies). 



As a Fern-plant occasionally arises directly from the tissue of the prothallu.s 

 without the intervention of the sexual act, so also spore formation is occasionally 

 omitted, and the prothallus arises vegetatively from the leaf of the sporophyte 

 (apospory in varieties of Athyrium, Jspuliitin, Aspleniuindimorphum, Nephrodium 

 2)seudo-mas. var. cristata apospora) ( 108a ). 



Sexual Reproduction 



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

 and female, are produced. Although neither alone is as a rule 

 capable of development, the actual reproductive body is formed by 

 the fusion into one cell of two such sexually differentiated cells. 

 The elements of the male and female nuclei remain for a longer or 

 shorter time distinguishable in the nucleus resulting from their 

 fusion ( 10J >). 



In fertilisation, as a rule, two uninucleate cells fuse, even when the vegetative 

 protoplasts are multinucleate. In Albugo Bliti, A. portulacae and A. tragopogcniis, 

 however, STEVENS found that the numerous nuclei of the ovum fused with an equal 

 number of sperm nuclei. Probably other cases of this kind may be found. 



The further development of non -nucleated fragments of the egg when united 

 with a spermatozoid has been shown to occur among animals by O. and R. HERTWIO, 

 BOVEKI, and DELA.GE. This phenomenon which is termed MEROGONY has also been 

 observed by WIXKLEII iu Cystoscira barbatn, one of the Fucaceae ( no ). 



In connection with parthenogenesis it was noted above that the incapability 

 of further development which characterises the unfertilised ovum may be over- 

 come by other means than copulation with the male cell ; KLEBS had already 

 shown this to be the case with the gametes of certain Algae. It is thus necessary 

 to distinguish in fertilisation between a stimulus, which removes the arrest laid 

 on the further development of the ovum, and the cell fusion, which influences 

 the natui'e of the resulting organism (amphimixis). The two influences are united 

 in the case of natural fertilisation ( m ). 



As it is thus necessary in sexual reproduction not only to provide 

 for the production of male and female cells, but also to ensure their 

 union, it becomes at once evident that, for sexual reproduction, the 

 organs must have a different structure than if they were designed 

 solely for vegetative activity. The sexual organs accordingly often 

 exhibit a special and peculiar form, and differ materially in appear- 

 ance from the vegetative parts of a plant. 



