172 THE STRUCTURE OF FLOWERS. 



" II etait done evident que les parasites avaient determine 

 les memes effets que les tubes pollmiques : raccroisement des 

 ovaires et des placentas et le developpement des ovules." 



The reader will here see the importance of this curious 

 instance as bearing upon my general theory of growth in 

 response to irritation ; so that if ovaries, placentas, and ovules 

 can be stimulated into growth and developraent, there is 

 no a priori reason why other parts of flowers may not equally 

 well grow in response to irritations set up by the insect 

 visitors ; as I have already shown to be the case in Clerodendron* 

 and in Mr. O'Brien's experiments. t 



Perhaps it will not be amiss to notice here a very similar 

 action of the suspensor in Orchids, described by M. Treub, 

 which grows "backwards," escapes from the micropyle, and 

 then ramifies in various ways, clasping and burrowing into 

 the ovarian walls like a parasite in order to convey nutritive 

 matters to the rudimentary pro-embryo. + 



Finally, M. Guignard remarks upon the degradations in 

 the essential organs of Orchids as accounting for the well- 

 known difficulty in raising seed from them : " Malgre le 

 nombre immense des grains formees dans les conditions 

 naturelles comme dans les serres, nombre qui parait etfe 

 d'ailleurs une signe de degradation physiologique dans une 

 famille oii la differenciation morphologiqne des organes floraux 

 est cependant si elevee, I'insuffisance de reserve alimentaire 

 contenue dans leur embryon microscopique, en necessitant des 

 conditions speciales pour le developpement, suffit peut-etre a 

 expliquer les difficultes et les insucces de la reproduction des 

 orcbidees par gi'aines, et la parcimonie relative avec laquelle 

 elles sont distribuees dans la nature." 



* See p. 130. t See p. 114. 



X Notes sur VEmhryog^nie de quelques Orchid^es, Verhandelingen der 

 Koninklijke Akadamie van Wetenschappen, 1879. 



