376 Darhishire . — Observations on Mamillaria elongata . 
Goebel and Ganong refer to the general biology of the Cactaceae, but 
we get only a superficial idea of the biological significance of the various 
organs and structures which characterize this plant-form. 
The tubercles, crowned by a set of spines, form a very conspicuous 
feature in the various species of Mamillaria , but the spines are generally 
very briefly referred to in the literature. Their function is usually, if not 
always, put down, without further discussion, as that of affording protection 
for the living plant-body against grazing animals. Ganong, Goebel, 
Delbrouck, and others have put forward this view. 
This explanation of the spines, which form such highly developed 
structures in many species of the Cactaceae, has never appeared to me to 
be quite satisfactory. A careful external study of the succulent plants 
at the Royal Gardens, Kew, still more increased my dissatisfaction with 
this explanation of their function. 
The following paper is an attempt on my part to get nearer a better 
understanding of that plant-form of which I have taken Mamillaria elongata 
as a typical representative. The problem I put before myself, when I 
began these observations in 1901, was this : what is the explanation of the 
plant-form represented by Mamillaria elongata , and what is } more especially , 
the meanmg of the spines which form so characteristic a feature of this 
plant ? 
A problem which has occupied numerous botanists is only referred to 
briefly in the following paper. This concerns the homology of the tubercles 
and spines of the Cactaceae. This question seems to have aroused far 
more general interest among botanists than their biological significance. 
Of course no detailed account of the biology or ecology of any plant- 
group can be quite satisfactory, which is not based on field-work in the 
native haunts of the plants concerned. 
For this reason great interest attaches to the establishment of a Desert 
Botanical Laboratory on which D. T. MacDougal reports in the Journal of 
the New York Botanical Garden of 1903 ( 18 , p. n). It is hoped that 
continuous observations will be made of conditions and plants, and with 
the material thus collected we should finally gain a clearer insight into the 
life of desert plants with which Volkens has already made us familiar to 
a certain extent. I have no doubt that many general physiological 
problems will be brought nearer a solution by being thus carefully studied 
in localities where the functions of the plant are carried out under adverse 
conditions. 
I have, however, attempted to give some physiological explanation 
of some of the structures met with in Mamillaria elongata , although I have 
never myself visited any of the tropical American deserts. 
It was first my intention to examine in detail thirty or forty different 
species of the Cactaceae, but unfortunately the time at my disposal would 
