FLORAL BIOLOGY. 205 



whereas, in reality, no branch of vegetable physiology offers 

 more, or more interesting, problems for solution. One factor 

 that has deterred many from work of this kind has doubt- 

 less been its laborious nature ; few persons are willing to 

 undertake experiments or observations lasting over several 

 years, preferring to do something which may be speedily 

 finished and put into print. 



With this introduction, we may go on to consider the 

 chief departments of the subject. Of the various treatises 

 above-mentioned that deal with the whole subject, that of 

 MacLeod is much the best for treatment of principles, 

 while Loew's works give all possible details. Mailer's 

 book, though a classic, is now very seriously out of date, 

 and is no longer suited for general readers or for students. 

 Much of what follows is based upon MacLeod's work ; it 

 is unfortunate that the book itself, being in Dutch, is avail- 

 able only to a limited circle of readers (it has a French 

 abstract). Starting with the more general ideas that under- 

 lie the whole subject, we shall deal first with 



CROSS-FERTILISATION AND THE KNIGHT-DARWIN 



HYPOTHESIS. 



As stated by Darwin, this is " that no organic being 

 fertilises itself for a perpetuity of generations, but that a 

 cross with another individual is occasionally — perhaps at 

 very long intervals — indispensable ". By his work on 

 orchids, Darwin rehabilitated Sprengel, and gave a fresh 

 impetus to the study of floral mechanisms. In the writings 

 of the earlier workers {e.g., in Hildebrand's Gescklechter- 

 vertheilwig, 1867) may be clearly seen the tendency to 

 explain all flowers as adapted in greater or less degree 

 to cross-fertilisation, whilst self-fertilisation is usually re- 

 garded as being actually harmful. This mode of looking 

 at the subject is natural enough when one takes as a 

 starting-point such a group of plants as the orchids, with 

 their extremely complex mechanisms for obtaining cross- 

 fertilisation. Darwin's experiments upon cross and self- 

 fertilisation show that the latter is not actually harmful in 

 itself, but that, in most cases, its offspring will, ceteris 



