XVIII 



THE PHYSIOLOGY OF REPRODUCTION 



239 



Death of, 

 Parent 



The Biological Importance of Sexual Reproduc- 

 tion. The biological importance of sexual reproduction cannot 

 be overestimated. As long as the surroundings of a plant are 

 favourable, vegetative reproduction suffices, but under unfavour- 

 able conditions, the life of the particular race of plants can only be 

 continued over the hard time by sexually produced bodies. The 

 seed produced as the result of sexual union is capable of retain- 

 ing its vitality under external conditions which would destroy 

 the mature plant. During a period of drought the vitality of 

 the young embryo within the seed is only suspended, not de- 

 stroyed, and when the ne- 

 cessary conditions again Germination 

 occur, it germinates and 

 produces a perfect plant. 



Life -History. All 

 the changes which a plant 

 undergoes from birth to 

 death are called its life- 

 history. The life-history 

 of an annual plant con- 

 sists of the germination 

 of the seed, the produc- 

 tion of the seedlings, its 

 growth to maturity, when 

 it flowers and produces 

 pollen and ovdles. The 

 generative nucleus of the pollen grain unites with the oosphere 

 in the ovule, and the oospore is formed. The oospore develops 

 into the embryo, and a seed is produced. The parent dies, 

 but the life of the particular race of plants is carried on by the 

 embryo in the seed. 



The life -history of a biennial plant consists'of the germination 

 of the seed and the production of the seedling ; its growth and 

 the storing up of reserve material, which ends the first 

 year of its life ; then during the second year growth recom- 

 mences, and a flower stem and flowers are produced ; pollen 

 and ovules are formed ; these produce seeds, and the parent 

 dies too. 



The life-history of a perennial plant takes three or more 

 years for its completion. It consists of the germination of the 



FIG. 234. Graphic illustration of the Life- 

 history of an annual plant. 



