538 LECTURE XXXI. 



from without, by means of which the oosphere is transformed into the embryo, and the 

 latter into a new organism. It will be the object of my last series of lectures, how- 

 ever, to go into this matter in detail : only the one point need now be noticed, that 

 not only in the Phanerogams but frequently in the Cryptogams also, the result of the 

 fertilisation of the oosphere is not merely its development into the embryo, but also 

 that other parts of the mother plant are thereby incited to further growth. In the 

 Phanerogams the formation of endosperm in the seed, the swelling of the ovary and 

 its development into the ripe fruit, are due to the action of the pollen : the infinite- 

 simal quantities of fertilising substance which are transferred by means of the pollen- 

 grains, e.g. of a Gourd-plant, to the female flower, are the cause of the development 

 from the ovary, by growth, of a fruit which is occasionally a hundred pounds in 

 weight. A still more remarkable case however in this connection, as Hildebrand^ 

 has shown, is presented by the Orchidese : at the time when the pollen reaches 

 the stigma, the ovules in the ovary, and which are to be fertilised, are either still 

 incomplete or have even not yet commenced to be developed at all, and if no pollen 

 is transferred to the stigma these essentially female organs of reproduction de- 

 velope no further. The growth of the pollen-tubes into the stigma acts as a 

 stimulus on the ovary, and the latter now begins to grow vigorously, and in it the 

 ovules only now develope, or even begin to be developed. 



In the case of the action of gravitation and of light we had to acknowledge that 

 the connection between cause and effect is entirely unknown to us, and in the case of 

 all the phenomena just mentioned we are no less compelled to say that all explanation 

 as to how the eflfects perceived result from the causes perceived is wanting. 



' Hildebrand, ' Die Fruchtbildung der Orchideen ein Beweis fur die doppelte Wirkung des 

 Pollens ' (Bot. Zeitg. 1863, pp. 329, &c.). 



