626 LECTURE XXII. 



which group the enlarged archegonium is termed the calyptra. 

 In connexion with this it may be mentioned that in various of 

 the Heterosporous Vascular Cryptogams (Marsilia, Salvinia, 

 Isoetes), in which the female prothallium produces in the first 

 instance only one or a few archegonia, the further growth of 

 the prothallium depends upon whether or not fertilisation 

 takes place : thus, if the first formed archegonium be fertilised, 

 the prothallium grows no more ; but if it fail to be fertilised, 

 the prothallium grows and produces one or more new arche- 

 gonia. In the Phanerogams even pollination, which is only a 

 preliminary process, affects the condition of the organs of the 

 flower ; it is well known that pollination causes flowers to fade, 

 and in some plants, notably in the Orchids, the development 

 of the ovules in the ovary does not take place until the flower 

 has been pollinated, so that it is clearly dependent upon the 

 stimulus of pollination. 



The spores of plants, whether sexually or asexually pro- 

 duced, may begin at once to develope into a new individual, 

 that is, to germinate, or they may pass through a longer or 

 shorter period of quiescence. Those which germinate imme- 

 diately on their formation have, as described above, a thin 

 wall, whereas those which are capable of passing through a 

 period of quiescence have a thickened cell-wall. In some 

 cases spores are incapable of immediate germination, notably 

 sexually produced spores. For instance, immediate germina- 

 tion is only known to take place, among the Algae, in the 

 zygospores of Botrydium and of Ectocarpus, and in the 

 oospores of Fucus. Among the Fungi, the oospores of the 

 Peronosporeae and of the Saprolegnieae pass through a period 

 of quiescence. 



The mode of germination of the spore is, as might be 

 expected, widely different in different cases. In most cases a 

 spore gives rise to a single individual, either by protruding 

 filamentous outgrowths which develope into the plant-body, or 

 by division to form a compact mass of tissue. In some cases 

 the spore behaves like a reproductive organ ; from its proto- 

 plasm are formed a larger or smaller number of cells, either 

 motile or non-motile, which are set free and are either sexual 



