276 Botany 



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, Straw- 

 berry, 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 Eanunculus Ficaria, which 

 reproduces itself by root tubers, are hardly able to produce good seeds. 

 They multiply exclusively by asexual methods without suffering any 

 degeneration. Continued reproduction by vegetative means used to 

 be regarded as necessarily injurious. 



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 Mould 

 Fungus (Mucor Mucedo), whose vegetative spores (conidia) are very 

 widely distributed, occasionally develops 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 to 

 induce the non- cellular Alga Vaucheria to produce at will either 

 non-sexual swarm-spores or sexual cells. In many plants unfavourable 

 external conditions apparently give the impetus to a sexual mode 

 of reproduction. The sexual product (zygospores of Algae) seems 

 better able than the vegetative germs (swarm-spores of Algae) to 

 remain a long time at rest, and so withstand the disastrous effects of 

 an unfavourable environment. No inference can be drawn, however, 

 from the function of the sexual germs in this instance concerning the 

 necessity for the existence of a sexual, in addition to a vegetative, 

 mode of reproduction ; for in other cases it is the vegetative re- 

 productive bodies, as, for example, the spores of Ferns and Horsetails, 

 which are especially equipped for a period of enforced rest. 



What makes digenetic reproduction essentially different from 

 monogenetic is the union of the substances of the parents and 



THE CONSEQUENT TRANSMISSION AND BLENDING OF THE PATERNAL 



AND MATERNAL properties. As special care is almost always taken 



