754 



LECTURE XLII. 



would be but a very lame one, since spores are not eggs at all; it is only in 

 the lowest regions of the vegetable kingdom, especially in the Fucacese and a 

 few other forms, that oospheres are separated from the mother-plant. 



From these preliminaries we may now proceed to the consideration of the 

 processes of fertilisation itself. Here however there are concerned so many organs 

 boxed one within the other, that it is scarcely possible to make clear all the parts in 

 their mutual relations in any one figure taken directly from Nature. In the interest 



of the reader, therefore, I prefer to 

 illustrate the essential points by 

 means of a diagram. 



Fig. 438 is a diagrammatic lon- 

 gitudinal section through the whole 

 ovule of a gymnospermous plant: 

 a is the integument, and k the mi- 

 cropyle, which in the case of the 

 Gymnosperms is relatively wide open 

 at the time when the pollen-grains 

 or microspores commence their 

 business of fertilisation. At this 

 time also a drop of fluid is 

 frequently excreted, and exudes 

 from the micropyle; the pollen- 

 grains carried here by the wind 

 stick to this, and when the fluid 

 contracts, the pollen-grains also are 

 drawn in with it and come to lie on 

 the projection of the nucellus of the 

 ovule b B. Such a pollen -grain 

 is seen at h. This process, namely 

 the. conveyance of the pollen-grains 

 to the female organ in the case of the 

 Phanerogams, is termed polUmtim, 

 and it occurs in many Gymno- 

 sperms at so early a time that the 

 true reproductive organs within the 

 ovule have not as yet even begun 

 to develope, and in the case of the Pines a full year may pass before the 

 fertilising tube of the pollen already contained in the closed micropyle reaches the 

 archegonia. 



The embryo-sac or macrospore arises in the Gymnosperms deep in the interior 

 of the nucellus of the ovule, and grows slowly up to a considerable size ; in the case 

 of the very large-seeded Cycadese it may attain a length of 2 cm. or more and a breadth of 

 1-5 cm. or more. It becomes filled with a succulent cell-tissue, the endosperm rfi, in 

 which (mostly only after a long time, and sometimes not before the seed appears to be 

 already nearly ripe) the archegonia e / are formed : these archegonia are of very 

 simple structure, though at the same time relatively large : in the case of the large- 



FIG. 438. —Diagram of the ovule of a Gymnosperm in longitudinal 

 section. 



