ESSENTIAL FLORAL LEAVES. IO9 



line. This condition is common among gymnosperms. A female 

 flower of yew, for instance, consists of a single straight ovule 

 terminating a short axis crowded with scale leaves. It is much 

 rarer among angiosperms. The solitary ovules of docks and 

 nettles are examples. Sometimes the nucellus, with its covering, 

 is bent or campylotropous, as in campion and shepherd's purse. 

 The commonest condition, however, is the inverted or anatropous 

 one. Here the ovule turns sharply down on its stalk, part of 

 which unites with the integuments, and forms a ridge or raphe. 

 Cross- section of larkspur, lily, or hyacinth ovaries will show 

 this welL The ovules of pinus, though without stalks, must be 

 considered anatropous, since their microp}des are downwardly 

 directed. 



It will be remembered that the flower was defined (p. 92) as a 

 spore- producing organ, and we have seen that a pollen sac is a 

 spore case or sporangium, producing numerous pollen grains or 

 spores. An ovule is another kind of sporangium, which produces 

 only one spore, called in this case an embryo sac, for reasons that 

 will presently appear. One of the cells of the nucellus in a 

 young ovule very early becomes larger than its neighbours. 

 This is the embryo sac. It does not, like a pollen grain, become 

 free, but is destined, if the conditions are favourable, to originate 

 a rudimentary plant or embryo within it, which, surrounded by 

 other structures developed from the ovule, will ultimately consti- 

 tute the ripe seed. The embryo sac in the mature ovule of the 

 angiosperm l (fig. 49) is no longer a simple cell ; it occupies a 

 considerable part of the nucellus, and its apex directly adjoins 

 the micropyle. In its interior is an abundance of protoplasm, 

 with large vacuoles, and a central nucleus, the embryo sac nucleus. 

 Six small cells are also contained within the embryo sac, three 

 at its apex and three at its base. The former are known as the 

 egg apparatus, the latter (being exactly opposite) as the antipodal 

 cells. Two of the cells composing the egg apparatus are smaller 

 than the third, and situated rather nearer the micropyle. They 

 are known as co-operating cells. The third and larger cell, the 

 ovum or egg-cell, is of greatest importance, since it gives rise to 

 the embryo. 



Proofs that the Flower is a Shoot.— I. The floral receptacle is a stem 

 because it bears lateral members, differing from it in shape, and developed 

 acropetally, just as in an ordinary shoot. The internodes are usually 

 suppressed, but this is a common occurrence in ordinary shoots. Some- 

 times, too, there is a distinct internode between the pistil and other mem- 

 bers, or between essential organs and perianth. It happens in some 



1 The scope of this work will not allow of reference to gymnosperms in this 

 connection. 



