39 2 Darbishtre. — Observations on Mamillaria elongata . 
Fungi, as I think they are in no way connected with the life of the plant. 
The parts which they infest are cut off completely from all cytoplasmic 
connexion with the living part of the plant. An investigation of their life- 
history would be of no interest except for mycologists. I therefore leave it 
to the latter to investigate the Fungus in question. 
3. Homology of the Tubercle. 
I have not considered it necessary or even important to investigate 
very fully the development of the different members and organs of our 
plant. But as I was not able to fully agree with some results obtained by 
other authors I did follow up the formation of the new organs at the 
growing-point of the shoot more in detail. 
The actual growing-point of Mamillaria elongata can best be examined 
by cutting a series of microtome sections in directions parallel and trans- 
verse to the longitudinal axis of the plant. The vegetative point of the 
main axis, however, is often very difficult to cut except with the hand- 
razor, owing to the hardness of the spines and the impossibility of removing 
the air from the lower ends of the older and larger spines. But the small 
lateral detachable shoots can be embedded in paraffin in toto, and they 
give fairly satisfactory serial sections. Their spines are not yet very hard, 
and by prolonged lying in absolute alcohol the air can be almost entirely 
removed, but the older parenchymatous tissues shrink to a great extent. 
The cells near the apex, being full of protoplasm, seem to remain fairly well 
preserved. 
The actual organic apex of the shoot is seen to be a very flat cone 
(PI. XXV, Fig. 18 ; PI. XXVI, Figs. 39 and 40). It is covered by a very 
distinct epidermis of fairly large cells. Further inside we get undifferen- 
tiated periblem, which is succeeded by a clear indication of the differen- 
tiation of the vascular strands of the plerome cylinder. A large-celled pith 
is soon marked off. 
Laterally on the apical cone of the main apex smaller conical pro- 
tuberances are being formed at very close intervals (w). They first appear 
as small outgrowths in the formation of which both dermatogen and 
periblem take an active part. We will follow out their development 
without discussing at present their homologies. Each of these new grow- 
ing-points in its turn grows out, and at first slightly overgrows one of 
the next young growing-points. At this stage the most actively growing 
part of the outgrowth is on that side of the hump furthest from the central 
growing-point (PI. XXVI, Fig. 40). The whole hump, which is simply 
a young tubercle, gradually becomes more differentiated (PI. XXVI, 
Figs. 40-43). The body of the tubercle consists of fairly large cells which 
do not look as if they were very actively growing (PI. XXVI, Fig. 45, w). 
The meristematically most active portion is the future cushion and its 
