2-i HISTORY OF BRITISH TERNS. 



■which lies downwards produces the root, and that part which 

 lies upwards produces the rudimentary stem. The spores 

 arc very minute cells of various shapes, mostly roundish, 

 and often heautifullv ornamented with markinirs on the 

 exterior. They consist merely of a small vesicle of cellular 

 tissue, and as they grow, this vesicle hecomes divided into 

 others, Avhich again multiply and enlarge, until they form 

 a minute green leaf-like patch, roundish, but irregular in 

 outline, and unilateral, forming a primordial scale, or 

 germ-frond, technically called the prothallus, on the under 

 side of which certain germ-cells are produced. One of these, 

 it appears, at length becomes the axis of development, and 

 produces a small leaf or frond, which is usually very differ- 

 ent in aspect as well as size from the mature fronds, and 

 is succeeded by others, which acquire by degrees the 

 characteristic features peculiar to their species. 



In annual Ferns the mature character is soon attained, 

 but in those of perennial habit, two or more years of 

 growth is required before they reach maturity, and a much 

 longer period is of course necessary to those which acquire 

 tree-like stems. They, however, in most cases soon begin 

 to assume something of their peculiar appearance, so that 

 by the time three or four of these young fronds are pro- 



