PREFACE 



Dr Turrill's volume on British Plant Life inihe New Naturalist Series (No. lo, 1948), 

 a book which may indeed be recommended as an excellent introduction to the present 



study. 



With regard to the illustrations a word of explanation should perhaps be offered as 

 to their purpose and use. In a group hke the Pteridophyta, where the technical 

 difficulties are so great that it has been my unfortunate lot to have to correct errors in 

 the work of almost every previous investigator, the attainment of accuracy has been a 

 primary task without which no vahd general conclusions could have been drawn. 

 For this reason the use of photography has assumed a special importance. The data 

 themselves have in the first place been assembled as far as possible according to the 

 principle that what cannot be photographed cannot be used as evidence. The photo- 

 graphic evidence so assembled has then been utilized to provide illustrations to the 

 book on a scale which should enable the reader to repeat for himself upon the printed 

 page enough of the observations quoted to judge of their validity. Only so can finality 

 in the establishment of basic facts be hoped for, and that many errors have been 

 removed or prevented by this procedure is certain. That some errors may still remain 

 is, however, only too probable, although where uncertainty is known to exist it is 

 unlikely to exceed the limits indicated in the text. 



All details regarding technical methods are given in the Appendix under the two 

 heads of cytology and photography. In cytology, more perhaps than in any other 

 science, progress depends on manipulative skill and that type of low cunning which is 

 needed to apply old methods to new uses. A considerable range of methods old and 

 new will be found illustrated throughout the book, from which it will perhaps be clear 

 that the shortcomings of previous workers, to which attention has so often to be directed, 

 have been due in the main to the shortcomings of their tools. Standard techniques such 

 as that of sections stained in haematoxylin or gentian violet are indispensable for certain 

 purposes, and when used with precision in easier material can yield all the information 

 required. This is not, however, the case in the Pteridophyta. Sections are indeed of 

 great value as supplementary evidence and for comparative and morphological 

 purposes, but except in very rare cases, the Osmundaceae and Selaginella being the only 

 ones known to me, sections alone are inherently unsuitable for accurate cytological 

 work. Only by the squash methods, either with Feulgen staining or with aceto- 

 carmine or other reagents, can accuracy be reached. In some groups with large 

 sporangia, such as the Osmundaceae, Eusporangiatae, Lycopods, Horsetails and so on, 

 smear or squash methods as devised for flowering plant stamens can be used without 

 modification. The Leptosporangiate Ferns, however, which form the subject-matter 

 for more than half the book, have sporangia far too small for this. With only 8 or 16 

 mother cells in each it is impossible to handle a sporangium singly, and the fact that 

 they grow in sori in which many developmental stages are always present together offers 

 what at first sight seems like an insuperable obstacle to the use of the simpler smear 

 methods. The success, such as it is, which is claimed in the present study is primarily due 

 to the overcoming of these preliminary difficulties as a result of which the power of new 

 techniques has become fully available, and it should be found that, undetected accidents 

 apart, in the main where accuracy is claimed, even in species like Cystopteris with n = 1 26, 



IX 



