vi PREFACE. 



positions taken up by dorsi ventral organs. It is the 

 general custom of those who have written on this 

 subject to speak of the normally lower surface of the 

 organ as the ventral, and of the upper as the dorsal. 

 Morphologists, however, term the normally upper sur- 

 face of dorsiventral leaves the ventral, and the lower 

 the dorsal. I have adopted the former use of these 

 terms with regard to leaves, so that in discussing 

 the positions of these organs, I mean by dorsal the 

 normally upper surface, and by ventral the normally 

 lower surface. 



I have to thank many friends for valuable hints 

 and kindly criticisms, more especially Prof. Michael 

 Foster, Sec. R.S., and Mr W. T. Thiselton Dyer, 

 C.M.G., F.R.S., Director of the Royal Gardens, Kew, 

 to both of whom I am much indebted. 



CAMBRIDGE, 



May, 1886. 



