84 ASSOCIATION OF ORGANISMS—THE WEB OF LIFE 
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, 
Pp. 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 
Z, afterwards developing into an embryo. 
PG 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 
Fig. 1078.—Diagrammatic Section througha in the former, since cross-fertilized 
aad 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:—(1) 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) Covo/éa, made up of 
petals, which are commonly conspicuous; (3) Thread-like S¢amens, 
within the thickened ends (azthers) of which is formed the pow- 
dery substance, fo/éen, from which the fertilizing living substance 
is derived; (4) Pes¢z2, consisting of one or more Carfels, 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 
Petal Pollen-Grains 
BW 
SS 
4 
Y 
4 
4 
QS 
