54 



THE GARDENER'S ASSISTANT. 



into contact with the ovule, enters the micropyle, 

 and impinges upon the embryo-sac. One of its 

 generative nuclei in some way enters the sac and 

 comes into contact with the female nucleus as 



Fig. 6fi.— Section of an Ovule, showins: the entry of the Pollen-tube 

 into the Embryo-sac. 



generative nucleus 



before stated, the other 

 shrivels and disappears. 



Where pollen falls on the stigma of the same 

 flower what is termed autogamy or close-fertiliza- 

 tion ensues, and this is necessarily the case in 

 flowers that do not open, and are hence called 

 cleistogamous. More often than not close-fertili- 

 zation is prevented by the circumstance that the 

 stigma of the flower yielding the pollen is not 

 mature at the time the pollen falls on it, and so 

 no germination of the pollen-tube takes place, or, 

 in any case, its male nucleus does not reach the 

 egg-cell. What generally happens is that the 

 pollen of one flower is deposited on the stigma 

 of another flower on the same plant, or another 

 plant of the same species. This is termed cross- 

 fertilization. The means by which the pollen is 

 conveyed from one flower to another are mani- 

 fold: sometimes it is carried by the wind, and 

 this is the explanation of the sulphur showers 

 sometimes spoken of, the so-called sulphur being 

 the pollen, generally of pine-trees, which is 

 produced very abundantly, and is blown in 

 showers by the wind. 



Insects are the most frequent pollen-carriers. 

 Attracted by the bright colours and fragrance 

 of the flowers, they visit the flowers for the sake 

 of the nectar they contain, or some of them for 



the sake of the pollen itself. In either case, if 

 circumstances permit, the insect readily conveys 

 the pollen to the stigma of the same flower or 

 to those of some other flowers. 



The astonishing diversity in the shapes of the 

 flowers and the varying relative lengths of the 

 stamens and styles are, many of them, as has 

 been pointed out, explicable by the fact that 

 they are "adaptations" which so facilitate the 

 access of insects and direct their passage in one 

 flower as to compel the unconscious insect to 

 remove the pollen, and in corresponding ways 

 to deposit it on the stigma of another flower 

 (fig. 67). This doubtless also accounts for many 

 curious forms of flowers, the hairs, the warts, 

 the grooves, the springs, the mechanical contri- 



Fig. 67 — Lythrum. Section of Flower showing two rows of Stamens— one 

 short, one long. The Style is short. 



varices, and the endless means by which insects 

 are compelled to go in the particular track best 

 adapted to secure the removal of the pollen and 

 its deposit on the stigma. Often the mechanism 

 seems to be adapted to one particular sort of 

 insect, and if that be absent or not forthcoming, 

 the flower cannot be fertilized. This is one 

 reason why certain exotic plants do not produce 

 seed in this country. 



Whilst the mechanism of the flower is in 

 many cases so developed as to necessitate the 

 co-operation of insects in order to secure the 

 production of new plants, in other cases analo- 

 gous mechanism is developed to exclude those 

 insects which are not wanted or which would be 

 injurious. This seems to be the meaning of those 

 glandular, sticky hairs by which the flower-stalk 

 or the calyx of some flowers is invested. A similar 

 purpose is served by a coating of hairs pointing 

 downwards, and thus preventing access of crawl- 

 ing insects. A brightly-coloured, fragrant, or 

 an " irregular " flower may always be considered 

 as adapted for insect agency. 



