Dar bishire. — Observations on Mamillaria elongaia. 395 
to be a leaf, and in its axil a lateral bud arises. Leaf and lateral bud grow 
up together and develop so as to form the cushion of tissue which gives rise 
to the spines or metamorphosed leaves. In the axil of a mature leaf- 
tubercle a lateral bud may be found. This, according to Goebel, is merely 
the result of a division into two of the original lateral growing-point which 
grew up with the leaf-base (12, pp. 77-84). 
Quite recently Rudolph has published some observations on the spines 
of Opuntia Missouriensis (23, p. 103- 109). He considers that in this 
species at any rate the spines are trichomes which have arisen in the axil of 
the leaf. He does not, however, wish to express any opinion concerning the 
other Cactaceae which he has not examined. 
It will be seen therefore that I do not quite agree with any views held 
by those who have examined the growing-points of Cactaceae. Referring 
to a figure by Goebel (12, p. 81, Fig. 41) which corresponds roughly to my 
Figs. 39 and 40 (PI. XXVI), I can only say that what Goebel calls the 
growing-point of the axillary shoot is to my mind merely the embryonic 
apical part of the leaf-primordium. The axillary bud is formed much later, 
and at least in M amillaria elongata has never been connected with the 
embryonic tissue which later on gives rise to the spines and which is 
supposed by Goebel to represent the axillary bud. 
The tubercle as a whole, in Mamillaria elongata at least, represents 
mainly the leaf-base, although its lower end may be partly derived from the 
stem-portion of the shoot. The spines represent the modified leaf-blade. 
Wetterwald’s observations on the Euphorbieae and Cacteae were 
interpreted by him in such a way as to confirm Goebel’s results. In my 
opinion his figures of Mamillaria coronaria (PI. XX, Figs. 33, 34) seem to 
agree quite well with my interpretation. 
Caspari was unable to detect any vascular tissue in the spines, and 
therefore considers the spines to be anything but reduced leaves (6, p. 6 ). 
But an examination of the young spine-primordia does show that there is 
at first a rough indication of a continuity of the plerome of the tubercle 
with the inner tissue of the future spine, at least in Mamillaria elongata 
(PL XXVI, Figs. 44, 45). Ganong also figures a spine of Opuntia coccinelli- 
fera with a delicate spiral vessel at its lower end (10, p. 9, Fig. 3). 
4. Physiology. 
(a) Introductory Remarks. 
As already mentioned in the introduction to this paper very little 
is known concerning the meteorological and other conditions under which 
the Cactaceae as a whole live. 
Our plant Mamillaria elongata is mentioned by Schumann ( 26 , p. 530) 
as occurring very extensively in the state of Hidalgo in Mexico. It has 
