162 THE JOURNAL OF BOTANY 
€ are apparently other and unknown causes which affect 
the distribution and recurrence of certain species. It sometimes 
happens that a form which has been very abundant for several 
h 
arent chan 
existed in the period of their abundance, yet close and frequent 
inspection of their formerly favourite habitats fails to reveal their 
presence. is the purpose of the fo owing communication to 
illustrate this statement, by recording examples of such phenomena 
at have been observed over an area within a radius of ten 
miles from Luton, which includes portions of Beds, Bucks, and 
erts. 
A remarkable example is that of Didymium Trochus, which was 
first observed in the spring of 1897 at Chaul End, Beds, and was 
figured and described in this Journal for 1898 (t. 886, fig. 1, p. 164) 
by Mr. Lister. During the fo lowing summer it was noticed in 
numerous stations. In October, 1897, Mr. C. Crouch first detected 
the plasmodium of the species in a heap of refuse taken from arable 
From 1897 to 1902 Didymium Trochus was of frequent occur- 
rence each summer and autumn, so much so that it ceased to be an 
object of special interest to local observers. In 1899 it was abun- 
dant also at Ivinghoe, Bucks, so that its known area of distribution 
in this district extended from Ivinghoe to Luton, ten miles west to 
east, and northwards to Pullox Hill, about eight miles. It was 
also in the early spring of 1897 that Mr. E. S. Salmon first 
nr; ; D. Trochus near Reigate, but in small quantity only (Journ. 
ot. 1. ¢.). 
ro 
record of its appearance. It was sought for diligently and with 
haunts being subjected to close inspection. Other forms were seen 
in plenty, but no D. Trochus, In July, 1905—that is, after an 
interval of three years—it was again found i good condition and 
fair quantity in a rick-yard at the foot of the Streatley Hills, Beds, 
well within the area over which it had been previously observed 
gai n at th 
é recorded in connection 
‘with Chondrioderma testaceum, Previously to 1894, the date of the 
er’s monog this species was recorded for 
, Scotland, and Flitwick, Beds; the latter 
y a few acres in extent, situated 
Lower Greensand. The 
