8 4 



ASSOCIATION OF ORGANISMS THE WEB OF LIFE 



-Petal Pollen- Grains 



Pollen-Tubes 

 Egg-Cell 



istil 



Nectary 



its study has done much towards revolutionizing the old cut-and- 

 dried method of studying botany, and has caused so many persons 

 to pay some attention to the world of life, that no excuse is made 

 for presenting here a few facts which will be familiar to most 

 readers, especially as the scheme of this work would be incomplete 

 without them. 



To understand the meaning of the word " pollination " it is 

 first of all necessary to say something, about the structure of 

 flowers. These are concerned with the production of seeds, in 

 each of which is to be found an embryo or plantlet in a dormant 



state, capable, under favourable con- 

 ditions, of growing into a new plant. 

 We have elsewhere seen (see vol. iii, 

 p. 335) that in animals the first stage 

 in propagation by means of eggs con- 

 sists in the fertilization of an egg-cell 

 or ovum by fusion with it of a smaller 

 cell or sperm, the fertilized egg-cell 

 afterwards developing into an embryo. 

 The embryo in a seed has also arisen 

 from an egg-cell, which has been fer- 

 tilized by material formed in the same 

 flower (self-fertilization) or in some 

 other flower (cross-fertilization). The 

 result in the latter case is better than 

 in the former, since cross-fertilized 

 egg- cells develop into more vigorous 

 embryos, and many floral arrangements are to be explained as 

 means by which cross-fertilization is brought about. 



Examination of a typical flower (fig. 1078) shows that it is made 

 up of four sets of structures, all of which are specialized leaves. 

 Beginning at the outside they are as follows: (i) Calyx, con- 

 sisting of a circlet of sepals, which may be green (as in a Butter- 

 cup), or brightly coloured (as in a Tulip); (2) Corolla, made up of 

 petals, which are commonly conspicuous; (3) Thread-like Stamens, 

 within the thickened ends (anthers] of which is formed the pow- 

 dery substance, pollen, from which the fertilizing living substance 

 is derived ; (4) Pistil, consisting of one or more Carpels, which in 

 the latter case may be either separate or fused together (a single 

 carpel is shown in the figure). The carpels contain a varying 



Sepal 



Stem 



Fig. 1078. Diagrammatic Section through a 

 Simple Flower 



