60 PLANTS 



133. The stamens may also be distinct or united into one 

 (monodelphous) , two (diadelphous) or more groups, the union 

 being due to the cohesion of either filaments or anthers. 



134. A union of floral organs occurs, not only between mem- 

 bers of the same series, but also between adjacent series. Thus, 

 the petals may be' united with the calyx cup in such a way that 

 they seem to spring from the edge of the cup instead of from 

 the receptacle. The stamens, likewise, may be adnate to the 

 petals or fused with the calyx tube and thus, like the petals, 

 apparently inserted upon it (perigynous) . Still greater fusion 

 may occur and calyx, corolla, and stamens all be more or less 

 united with the ovary and thus apparently inserted on its side 

 (perigynous) or top (epigynous) instead of on the receptacle. 

 In the latter case the ovary is said to be inferior. 



Function of the Flower 



135. The function of the flower is to produce the seed, but 

 this is accomplished only by the conjoint action of pollen and 

 ovule. Under normal conditions, the ovule at a certain time 

 begins a series of developmental changes by which it finally 

 becomes a seed. This latter phase of its development is begun, 

 however, only after pollination and fertilization. Pollination 

 is the transfer of pollen from the anther to the stigma by the 

 wind, by insects, or through some other agency. After reaching 

 the stigma the pollen grain develops a tubular outgrowth which 

 penetrates the tissues of the stigma and style growing down 

 to and into the ovule. A certain nucleus of the pollen tube 

 then fuses with a similar nucleus of the ovule. This fusion of 

 elements from the pollen grain and ovule is known as fertili- 

 zation, because, as a result of it, the ovule is stimulated to 

 further development which finally results in the seed, whereas, 

 if the fusion does not occur, there is no further development of 

 the ovule and no seed is produced. 



