778 LECTURE XLIII. 



with the oogonium genetically, but lie sufficiently near it to be influenced by it. 

 Of course the distance is not great, and De Bary states that it amounts to about 

 the diameter of an oogonium. In other cases again, the young oogonia exert a 

 modifying effect on the growth of such tubes on which male organs (antheridia) may 

 be formed. ' As soon as a vigorously growing lateral branch of this kind,' says De 

 Bary, ' attains a certain distance from a young oogonium, its end is seen to bend 

 over towards it, and then develope into an incipient antheridium.' He then con- 

 tinues, ' the divergence of the lateral branch described cannot be referred to any 

 other cause than special properties of the oogonium.' 



To my mind there can scarcely be a doubt that the pollen-tube of the 

 Phanerogams also is specially influenced by something which induces it to 

 grow with its end containing the fertilising substance directly towards the 

 opening of the very narrow micropyle, to penetrate eventually to the oosphere. 

 It is remarkable that this matter has as yet scarcely been thought worthy of 

 investigation. When one reflects how extremely inaccessible the micropyle of the 

 ovule in the cavity of the ovary usually is, how narrow it is, how great the distance 

 often is which the pollen-tube must traverse from the stigma through a long 

 style down to the ovary, and when one further considers that for the fertilisation 

 of any ovary which contains only one ovule, even a very few pollen-grains on the 

 stigma ensure that a pollen-tube reaches the micropyle (e. g. this is conspicuously 

 the case with Mirahilis longiflord), further, that on the stigma being sufficiently 

 pollinated, often hundreds and thousands of ovules in an ovary each receives its 

 pollen-tube, and when one further observes how the pollen-tubes of some Orchids 

 grow down free through the cavity of the ovary to the ovules, and so on, it must 

 be obvious that the entrance of the pollen-tubes into the micropyles can by 

 no means be a matter of chance, but that definite arrangements must exist 

 which lead the growing end of the pollen-tube to its destination. It is true the 

 tissue of the stigma and of the style are especially suited to at least offer no 

 hindrance to the pollen-tubes on their way to the cavity of the ovary ; moreover, 

 in many cases special relations of organization exist on the inner walls of 

 the ovary, which evidently have the purpose to show the pollen-tube the way, so 

 to speak, which conducts it to a micropyle. But why do the pollen-tubes when they 

 germinate on the surface of the moist stigma, grow directly into its tissue ; why do 

 they bend from the stigmatic lobes into the conducting tissue of the style; and why 

 do they follow in the ovary the indicated road-marks where space and opportunity 

 exist for deviations, and so on? It appears to me that in addition to the visible 

 coarser relations of organisation- referred to, invisible arrangements and unknown 

 forces exist which chiefly determine that the pollen-tubes find their way from the 

 stigma to the micropyle. 



