394 Dor bishire. — Observations on Mamillaria elongata. 
3*5 mm. at the other. At this stage they would be red in colour for about 
| or | the way from their upper end downwards. The central spine may 
now be distinguished by being slightly broader and redder in colour than 
the others. A few small protuberances from the epidermis of the spines 
are to be noticed now. But these develop more later on. The spines 
are subsequently, through the activity of the cork cambium, unfolded in 
the typical spreading fashion. This change in position is accompanied 
by the appearance of air in the lower hard fibrous cells of the spines 
(PI. XXV, Fig. 1 8). 
The points of the spines still remain reddish brown for a time, but 
later this colour also disappears. The small outgrowths from the epidermis 
of the spines are found almost exclusively on the inner and middle portion 
of the marginal spines, but all round on the large central one. The whole 
set of spines remains permanently in a slight depression ; at any rate the 
circular mound of tubercle tissue keeps pace with the growth of the whole 
spine-cushion (PI. XXV, Fig. 15). 
What then is to be said about the morphological nature of the tubercle 
and the spines ? 
The first small conical projections near the growing apex arise just 
in the way leaves would arise. To my mind there is no doubt that 
they represent leaves, that is leaf-primordia (‘ Blattanlagen ’). The leaf- 
primordium grows, and at the top of it there appear a number of small 
protuberances, new actively growing portions (PL XXVI, Figs. 40, 41, 42 v). 
What do they represent ? I have no doubt that the body of the primordium 
at this stage represents the leaf-base and the small protuberances represent 
the leaf-blade. The leaf-blade does not develop except to form the spines, 
and the leaf-stalk does not develop at all. The leaf-base develops most, 
but in the mature plant the tubercle may possibly represent in addition 
to the leaf-base, but to a limited extent, certain portions of the shoot which 
have become fused with it. 
The leaf-nature of the tubercle-primordium becomes clearer still, when 
we notice how in its axil, a short distance behind the growing-point of 
the shoot, a small lateral bud is formed (PL XXVI, Fig. 46, y). This lateral 
bud, however, is formed more on the lowest portion of the base of the 
leaf-primordium than on the main shoot itself. 
In Mamillaria elongata , at any rate, I can therefore see in the mature 
tubercle only the highly developed leaf-base. The spines together repre- 
sent the leaf-blade, the leaf-stalk being absent. This view does not agree 
with that expressed by most other authors who have examined the 
morphology of the tubercle of the Cactaceae. Ganong briefly summarizes 
these views ( 10 , p. 45). Kauffmann considers the spines to be leaves, 
Vochting and Delbrouck look on them as emergences, in which point they 
agree with Schumann. Goebel makes out the whole tubercle at first 
