390 BOTANICAL GAZETTE [November 



The principal interest centered in B. obliquum and B. dissectum, 

 which is often regarded as a variety of B. obliquum. There are 

 other forms which taxonomists describe as varieties of B. obliquum 

 and which may be as distinct and may have as definite a relation 

 to the parent form as we believe B. dissectum has to B. obliquum 

 but we did not make any study of these forms, and in making plots 

 and in counting we recognized only B. dissectum, and put all the 

 rest — the varieties oneidense, tenui folium, and elongatum — -under 

 the general name B. obliquum. Besides these varieties, which can 

 often be identified with a manual, there are fluctuating variations, 

 so that one who is not a professional taxonomist is tempted to 

 call the whole assemblage B. obliquum and let the name cover 

 B. obliquum and its derivatives. 



B. obliquum does not occur in such large numbers as B. vir- 

 ginianum, the plants of a group being more scattered, with seldom 

 more than a dozen plants on i sq. m. This difference in numbers 

 and the difference in grouping is indicated in fig. i. This plot 

 represents an area of 33 by 40 m. The dots and the crosses of the 

 diagram are all of one size, but the plants varied from sporelings 

 still attached to prothallia up to large specimens. Where plants 

 are so numerous, as indicated in the denser groups, not more than a 

 quarter of them have fruiting spikes. 



Why plants of B. virginianum should be so much more numerous 

 than those of B. obliquum, when they are growing in the same 

 situation, particularly when growing on the same spot, as shown 

 in the lower right hand group in fig. 1, is not obvious. A million 

 spores would be a very conservative estimate for the output of an 

 average plant of either species, and in the largest plants the output 

 probably reaches five or six million spores; but a comparison of 

 the number of plants and the number of spores would indicate that 

 far less than one spore in a million produces a plant which can be 

 seen above ground. One might guess that spores which do not sift 

 down immediately to a safe depth die very soon from exposure or 

 only a little later from the winter's cold; but we have noticed that 

 in the tropical rainy forests of southern Mexico, where Botrychium 

 is abundant and where there would seem to be no danger from 

 dryness or cold, prothallia are as difficult to find as in the United 



