REPRODUCTION 353 



Because of its complexity it seems to be difficult to subject the 

 process to a detailed physiological analysis. Only a brief men- 

 tion will be made here of the most important achievements in this 

 field. The consideration of fertilization will also be limited to the 

 angiosperms, and the morphological and cytological side of the 

 process will not be touched upon, since they are sufficiently de- 

 scribed in textbooks of plant morphology and cytology. 



Fertilization is preceded by the development of flowers which 

 are metamorphosed shoots. Instead of green leaves, these shoots 

 bear pistils, stamens, and a perianth. These organs are but little 

 or not at all active in photosynthesis, but they have a very high 

 respiratory rate. Therefore, in order that the formation and devel- 

 opment of flowers, particularly in large numbers, may take place, 

 a considerable amount of organic substances must accumulate in 

 the plant. Consequently, in annual plants blossoming usually 

 occurs after the middle of the vegetative period. In perennials, 

 especially in a large number of trees, blossoming, on the contrary, 

 takes place in early spring at the expense of organic reserves which 

 have been accumulated and stored during the previous year. 



Not being involved in the process of assimilation, the petals 

 and sepals do not require much aeration and therefore have but 

 few stomata. Hence, in spite of a thin cuticle, flowers always 

 transpire much less than leaves. This explains why cut flowers 

 wither so slowly. 



As a rule, flowers grow and develop rapidly. Hence, as has 

 been noted, even the opening and closing of flowers is of the nature 

 of growth movements and not of alternating movements. Because 

 of an almost continuous growth, flowers respire very intensively. 

 The more massive flowers and inflorescences, such as the spadices of 

 certain aroids, show a considerable rise in temperature as a result of 

 a high respiratory rate. 



Fertilization in higher plants is preceded by pollination, the 

 transfer of pollen from the anther to the stigma. In the process 

 of germination of pollen on the stigma , the external coat of the 

 pollen grain (exine) is ruptured in definite places, while its inner 

 coat (intine) is stretched into a long tube which descends through 

 the style, thus reaching the ovule. The germination of pollen is in 

 many ways similar to the germination of spores of fungi. Similar 

 to fungal hyphae, pollen tubes show a pronounced chemotropism. 

 The pollen of the majority of plants can germinate not only on the 



