142 THE STRUCTURE OF FLOWERS. 



flowers, there are many reasons for inferring their existence 

 to be due to the direct and irritatinsf action of insects them- 

 selves when searching for juices as food or otherwise. 



That a merely mechanical irritation may cause a flow of 

 nutrient fluid to the spot, so that the tissues may increase in 

 size by the development of cells, which would not otherwise 

 occur, is abundantly evident. It is seen, for example, in the 

 gi^owth and development of galls ; of the so-called " Ant- 

 plants " on Myrmecodia (p. 115), Acacia sphcerocephala, etc.; 

 in the thickening of all climbing- organs as soon as the irrita- 

 tion of the foreign body has commenced ; hence the inference 

 that hypertrophy may occur wherever an insect's pi'oboscis can 

 irritate the floral organs, is by no means without foundation. 

 Why the cell-contents of nectaries should especially give rise 

 to sugar, is a question at present beyond answering. Those 

 of conducting tissues appear to do the same. In the case of 

 nectaries it may, perhaps, have originated as a pathological 

 phenomenon which has become fixed and hereditary ; for 

 pathological conditions often determine a flow of gum, as in 

 Cherry-trees, resins in the Coniferce, watery and sugary dis- 

 charges from wounds, etc. ; and it is impossible to draw any 

 hard-and-fast line betv/een a pathological and varietal state : 

 as, for example, in closing the scar after the fall of the leaf 

 the fibro- vascular bundles are sometimes stopped by gum — 

 a process which, in this ease, might be regarded as normal, 

 and not pathological as in the former. 



If a particular locality be perpetually inntated, so to say, 

 for generations, all analogy shows that the effect may become 

 permanent and hereditary ; at least, as long as the irritation 

 is persistently renewed year after year. And, on the con- 

 trary, the theory is equally supported by the negative evi- 

 dence of the disappearance of the honey-glands Avhenever the 

 whole flower degenerates and becomes regularly self- fertilising 



