OEIGIN OF CONDUCTING TISSUES. 165 



three pollen-tubes are seen in section. Fig. 50, 6, shows the 

 formation of conducting tissue at the angle of the inflected 

 carpellarj edges of Buhus. The epidermal and subjacent 

 cells form the conducting tissue in this case. The cells on 

 the outskirts are charged with sphaera.phids. Fig. 50, c, is a 

 section of the ova-ry of a Crucifer. The replum or false dissipi- 

 ment, as in the Papavei'acece, forms the machinery for conduct- 

 ing the tubes. The dotted lines show the original lines of 

 fusion. Now, if my theory be true, that no structure exists 

 which has not been brought into existence through some 

 foreign action having been brought to bear upon it — either 

 directly from without, as insect agency, light, etc., or 

 indirectly through nutrition within the plant, — then, the 

 existence of this specialized tissue would never have arisen 

 had it not been for the iiTitating action of the pollen-tubes. 

 The analogous influence of the mycelium of a parasitic 

 fungus here gives us the clue. As such causes hypertrophy 

 to set in, and induces nutritive matters to accumulate upon 

 which the fungus lives, — just as the irritation of the egg or 

 pupa of a cynips or other insect causes a similar accumula- 

 tion of richly nutritive substances to be made within the 

 tissues of the gall upon which it feeds, — so the germinating 

 power of the pollen-grain and the growth of the pollen- 

 tube have actually brought about the formation of these 

 highly nutritive conducting tissues of the style. The effect 

 has then become hereditary, so that they are now in course 

 of formation, at least, during the development of the flower 

 in preparation for the ingress of the pollen-tubes. 



The remarkably stimulating action of the pollen-tube had 

 been observed more especially in Orchids. Hildebrand 

 noticed that the influence of the pollen was twofold, in that 

 it determined the growth of the ovary and the complete 

 formation of the ovules before the process of fecundation had 



