8 A. R. Horiuood — Rock-soil and Plant Distribution. 



Pig. explanation OF PLATE II. 



1. Herpctopora anglica. A piece of zoarium showing the method of branching 



typical of the genus Herpctopora. Of the ten zooecia shown four 

 are sealed and three are renewed zocecia. x about 11 diameters. 

 The type-specimen ; British Museum specimen No. D. 11359. Senonian, 

 top of Micraster cortestudinarium zone or base of M. coranguinum 

 zone. Chatham, Kent. W. Gamble Collection. 



2. H. anglica. Pieces of two branches of a zoarium, more highly magnified 



than Fig. 1. One zooecium is sealed, three are normal, and the 

 remaining zooecium is a heteromorphic individual of unknown function. 

 X about 27 diameters. Part of the same specimen as Fig. 1. 



3. H. anglica. A single normal zocecium still more highly magnified than 



Fig. 2. X about 33 diameters. The zooecium is the same as that at 

 the lower right-hand corner of Fig. 1. 



4. H. clavigera. A piece of zoarium magnified approximately to the same 



extent as Fig. 2. One sealed and three normal zooecia are shown ; 

 two of the latter are much broken. x about 25 diameters. The 

 type-specimen ; British Museum specimen No. D. 20607. Senonian, 

 zone of Belemnitella mucronata. Norwich. T. G. Bayfield Collection. 



5. H. clavigera. A single normal zooecium from another part of the 



specimen from which Fig. 4 is drawn, magnified to the same extent as 

 Fig. 3. X about 30 diameters. 



6. H. danica. A piece of zoarium magnified approximately to the same 



extent as Figs. 2 and 4, bearing four normal and one renewed zooecium. 

 X about 24 diameters. The type-specimen ; British Museum specimen 

 No. D. 19429. Danian. Faxe, Seeland, Denmark. J. W. Davis 

 Collection. 



7. H. danica. A single normal zooecium from the zoarium of which Fig. 6 



is a part, magnified approximately to the same extent as Fig. 6. 

 X about 24 diameters. 



III. The YALUE of a knowledge of the KoCK-SOIL DlSXKIBTJTIOIf 



OF Plants in tracing Geological Boundaries. 

 By A. R. HORWOOD. 



THOUGH ecology has been studied for a lonj; period on the 

 Continent it is only within the hist ten years that it lias become 

 an important branch of botany in this country. Now, however, it 

 is one of the most ardently pursued sciences, and has made great 

 progress. That branch of it which deals with soil factors, or the 

 distribution of plants upon different soils and a knowledge of the 

 nativity of plants upon particular soils, has received considerable 

 attention, and sufficient data have been accumulated to state the 

 principle that each rock-soil supports a characteristic flora: so that 

 the type of soil may be regarded as an index to the type of vegetation 

 of any district wliere that rock-soil is dominant ; and in the same 

 way it is possible to state by a recognition of the type of vegetation 

 the character of the rock-soil. 



Whilst emphasizing these main features it should not be forgotten 

 that exceptions occur, and that composite rock-soils are liable to 

 obscure the clear evidence afforded by self soils or those that are 

 pure. But they do not by any means invalidate the importance of 

 these main principles. The influence of man, largely through agri- 

 culture, traffic, industry, building, etc., upsets in some cases the 

 normal features noticeable in an undisturbed state of nature. But this 

 is inevitable, and must be taken into account in all biological work. 



