4 PROF. JULIUS MACLEOD ON TEN 
Despite this fact, innumerable constants have been determined by 
physicists and chemists. In a modern Dictionary of Chemistry we find 
hundreds of thousands of numbers which have been calculated during more 
than a century. Many of them (for instance, the atomic weight of the 
elements) were at the beginning only approwimate and have been corrected 
later on. It is certainly possible to carry out a similar work with animals 
and plants. 
My address seems to have made on some persons the impression that I am 
complicating things which are rather simple. In realitv, the method 
enables us to kace and to surmount difficulties which actually exist and are 
ordinarily overlooked. When we are told, for instance, that in Mnium 
rostratum the cells of the leaves are larger than in Mniwm serratum, this 
information is on the whole correct. But we may find specimens of rostratum 
the cells of which are really smaller than those of certain specimens of 
serratum! Such facts bring about hesitation and doubt. When we say 
that the breadth of the cells at the place of the yreatest breadth of the leaf, in 
the longest leaf of a fertile stem of M. serratum, varies between 16 and 26 p, 
and that the same character varies in rostratum between 22 and 36 p, we 
have replaced, it is true, a simple notion by a complieated one, but the 
given information is complete. The student is no longer disconcerted 
by the disagreement between the descriptive text and the faets he is 
observing. 
The material used for obtaining the measurements that follow was for the 
most part obtained from the Barker collection of British Bryophytes. 
This herbarium was given to the University of. Manchester by the late 
Thomas Barker, Professor of Mathematies, who was the founder of the 
Chair of Cryptogamie Botany in the University of Manchester. 
A few specimens were taken from the Manchester Herbarium. A 
number of rare specimens were kindly given to me by Mr. H. N. Dixon, 
M.A., F.L.S., to whom I offer here my best thanks. 
To Prof. W. H. Lang, in whose laboratory the present work was carried 
out, and to Prof. F. E. Weiss I am deeply indebted, as they made this work 
possible for me, and I wish to express to them my gratitude. 
I beg Prof. G. Unwin, who kindly helped me with reference to the 
language, to accept my most sincere thanks. 
"T 
