298 BOTANY PART I 



the formation of cells or cell-bodies which, after their separation from 

 the parent plant without undergoing any further change, either 

 germinate at once, or develop into new organisms after a period of 

 rest. This mode of reproduction, in which the growth and develop- 

 ment of the parent plant are directly continued, is also distinguished 

 as MONOGENETIC, VEGETATIVE, or ASEXUAL reproduction. The 

 special peculiarities and laws of growth of the parent plant are in 

 this case directly transmitted to the offspring. 



In SEXUAL REPRODUCTION, the second of the two modes of 

 reproduction, two kinds of reproductive cells each of which carries 

 the characters of the organism producing it are first formed, 

 but neither is directly capable of further development, and both 

 perish in a very short time, unless opportunity is given for their 

 fusion with each other. Not until one cell (the female) has fully 

 taken up and become inseparably united with the other cell (the male), 

 does it acquire the capacity of development and growth. This mode 

 of reproduction is designated SEXUAL or DIGENETIC reproduction. 



The physiological significance of sexual reproduction is not at 

 once apparent. In many plants the vegetative mode of reproduction 

 is sufficient to secure the necessary multiplication of the species, so 

 that plants are able to continue without sexual reproduction. Many 

 Fungi, for instance, are reproduced only vegetatively ; the cultivated 

 Banana, many Dioscoreaceae, and varieties of the Grape, Orange, 

 and the Strawberry, no longer reproduce themselves sexually, but are 

 propagated solely in a vegetative manner. The Garlic, which forms 

 small bulbs in place of flowers, the White Lily, and ReunuMuihu 

 Ficaria, which reproduces itself by root tubers, are hardly able to 

 produce good seeds, which can, however, be obtained by operative 

 interference, e.g. on separated inflorescences. They multiply ex- 

 clusively by asexual methods without suffering any degeneration. 

 Continued reproduction by vegetative means used to be regarded as 

 necessarily injurious ( 102 ). 



Since monogenetic reproduction is sufficient for the preservation 

 of the species, sexual reproduction must answer some purpose not 

 attained by the vegetative mode of multiplication, for otherwise it 

 would be altogether superfluous that the same plant, in addition to 

 the vegetative, should also possess the sexual form of reproduction, 

 which is so much more complicated and less certain. Even the 

 common Moulds, whose vegetative spores (conidia) are very widely 

 distributed, occasionally develop sexual reproductive cells in specially 

 formed sexual organs. 



In many of the lower plants (Algae and Fungi) it has been shown 

 that the development of sexual cells is dependent upon definite 

 external influences. KLEBS has demonstrated, in fact, that it is 

 possible by regulation of the external conditions (nutrition, 

 temperature, transpiration, composition of substratum and of the 



