644 



LECTURE XXIII. 



attention to certain other points. In the first place, we may 

 enquire why it is that plants reproduce themselves by means 

 of spores ; for, as we have seen, their somatic cells so generally 

 possess the reproductive capacity that the necessity for the 

 production of specialised reproductive cells may well be 

 questioned. There can be no doubt, however, that the for- 

 mation of spores is of great biological importance in main- 

 taining the existence of the various kinds of plants. Spores 



Fig. 75 (after Strasburger). Fertilisation in an Angiosperm (Monotropa Plypopitys], 

 In A, the male and female pronuclei are present in the oosphere o. In B, 

 the male and female pronuclei have nearly completed their fusion ; the 

 nucleoli have not, however, quite coalesced. 



are capable, namely, of retaining their vitality under external 

 conditions, such as long drought, lack of food, extremes of 

 heat or cold, etc., which would prove fatal to the individual 

 plant. But this property is also possessed in a high degree 

 by the variously modified buds (bulbs, bulbils, gemmae, corms) 

 which subserve vegetative reproduction. But spores afford 



