306 FOURTH GROUP. 



form a case for them, the ovary, and to provide the apparatus, the stigma, for the recep- 

 tion of the pollen. This great variety in the morphological significance of the carpels 

 is clearly seen by comparing the genera Cycas ^.^Juniperiis ; in Cycas the carpels are 

 like the ordinary leaves of the plant, and the ovules which are entirely free and exposed 

 are formed on their margins ; mjiirriperus the ovules are formed in the axils of the floral 

 leaves, which swell up after fertilisation and envelope the seeds in a pulpy substance, 

 the berry-like fruit of the plant. In the Primulaceae the ovules spring from the elon- 

 gated floral axis itself, and are enclosed at the time they are formed in a case, the 

 ovary, which is composed of the carpels and bears the stigma on a stalk-like prolon- 

 gation at its upper end. In most other Dicotyledons and Monocotyledons the ovules 

 are placed on the incurved margins of the carpels which have united to form an ovary, 

 and in this case therefore both produce and contain the ovules. With these very con- 

 siderable morphological differences the carpels agree physiologically in being always 

 excited to further development by fertilisation and during the formation of the seed, 

 and in participating to some extent in its fortunes. 



Pollination and fertilisation. In the interaction of the pollen and the oosphere 

 previously formed in the embryo-sac of the Phanerogams there are two points of chief 

 importance which must be carefully distinguished from one another, pollination and 

 fertilisation. Pollination is the conveying of the pollen from the anthers to the stigma in 

 Angiosperms, or to the nucellus in Gymnosperms ; there the pollen is detained by a 

 viscid substance, often also by hairs, and impelled to the emission of the pollen-tube. 

 In the Gymnosperms this tube at once pierces through the tissue of the nucellus, but 

 in the Angiosperms it grows downwards through the tissue of the stigma and through 

 the style, which is often of considerable length, till it reaches the ovule, when it makes 

 its way through the micropyle to the embryo-sac ; it is not till it is in contact with the 

 embryo-sac, and in the Gymnosperms has penetrated still farther, that the fertilisation 

 of the oosphere is effected J . Between the two processes of pollination and fertilisation 

 a long period of time, sometimes months, may elapse, but in many cases only days or 

 hours. 



Pollination is rarely brought about by the wind only (anemophilons flowers) ; where 

 this is the case large quantities of pollen are produced, in order to secure the desired 

 result, as in many Coniferae ; in a few cases only is the pollen thrown on to the stigma 

 by the bursting of the anthers, as in some Urticaceae ; insects are the means usually 

 employed to effect pollination (entomophilous flowers). For this purpose special and 

 often highly complicated arrangements are devised to allure the insects, and induce 

 them to visit the flowers ; and means are at the same time employed to ensure that 

 the pollen of one flower shall as far as possible be always conveyed to the stigma of 

 another flower, even in the case of hermaphrodites. It is with a view to these objects 

 that the parts of the flower assume definite forms and positions, which we will not go 

 further into at present, but only observe that insects are attracted to the flowers chiefly 

 by means of the nectar which is secreted in them ; this usually sweet juice is in 

 most cases produced deep down between the leaves of the flower, and the shape or 

 the parts of the flower is usually so contrived that the insect in searching for the 

 nectar must put its body in certain positions, and in doing so brushes pollen from 

 the anthers, which it afterwards deposits on the stigma of another flower. The 

 variety in the forms of flowers is chiefly due to these conditions, though the plan on 

 which they are all constructed is a comparatively simple one. The organs which 

 secrete the nectar, the nectaries, are therefore of special importance to the life of most 

 Phanerogams, but at the same time they are usually very inconspicuous, and in spite of 



1 [Strasburger, Neue Unters. ii. d. Befruchtungsvorg. b. d. Phanerog. 1884, has shown that the 

 growth of the pollen-tube through the tissue of the style is comparable with the growth of the hyphae 

 of a parasitic Fungus, in some cases penetrating the cells of the stigmatic surface. See also his 

 Botanische Practicum.] 



