EEPEODUCTION 613 



other gametophytes ; the buds of the ferns, the cuttings or buds 

 of the Phanerogams develop into new sporophytes like their 

 parents. Alternation of generations is thus left altogetlier un- 

 affected by vegetative reproduction. 



Some curious cases of this vegetative method are known. 

 Thus in the embryo sac of Ccelehogyne there is no fertilisation 

 of the oosphere, but still one or more embryos arise. This is 

 caused by a vegetative budding of certain cells of the nucellus, 

 which grow into the interior of the embryo sac and develop 

 into embryos. 



A further peculiarity of vegetative reproduction is that the 

 new individual is developed continuously on its origination. 

 There is no resting period, such as we find in most oases to mark 

 the behaviour of the more specialised reproductive cells to be 

 discussed below. 



Besides vegetative reproduction we have two cases of the 

 production of special cells to subserve this purpose. In the 

 first of these cells are differentiated which, after a longer or 

 shorter period of quiescence, can grow out into new plants. 

 These cells, possessing in themselves the power of producing 

 new individuals, are known' as asexual cells. They are differ- 

 ently named according to the phase of the plant on which they 

 originate ; those borne upon the sporophyte being called spores, 

 those on gametophytes gonidia. There is, however, no further 

 difference between them ; indeed, by many writers the name 

 spore is applied to both. 



The fact that they do not usually germinate till after a period 

 of rest, though this is often not very prolonged, suggests that 

 they originated in a need for the plant to develop certain cells 

 which should possess the power of passing through times of un- 

 favourable conditions without destruction. Such unfavourable 

 conditions would be likely to kill the more delicate vegetative 

 reproductive bodies. This view is supported by the fact that 

 many of thelower plants, particularly Yeast, do not produce gonidia 

 when conditions are suitable for the hfe of the ordinary indi- 

 vidual, but can be made to do so by cultivating them under 

 adverse conditions of moisture, food supply, &o. 



The gonidia which occur upon the gametophytes of the Algae 

 and Fungi are prodviced in various ways ; they may occur 

 singly or in large or small numbers in the interior of particular 

 cells known as gonidangia ; they maybe produced by abstricticn 

 from a cellular outgrowth of the thallus ; in this case again the 

 number produced from a single cell may vary within wide 



VOL. II. L L 



