Dar bishire. — Observations on Mamillaria elongata . 393 
spines. This part has gradually been carried up away from the growing- 
point (PL XXVI, Figs. 39-42, v). The active cushion is marked off by 
a rim which runs right round the upper end of the tubercle (PL XXVI, 
Fig. 43). Finally, the meristematic cushion-tissue is carried up to a position 
transverse to the longitudinal axis of the plant, through the inner side of the 
tubercle growing more rapidly than the outer one. The embryonic tissues 
of the cushion gradually show a number of well-developed conical pro- 
jections, which represent the future spines (PL XXVI, Figs. 41-43, r). 
They are made up of dermatogen and periblem layers at first (PL XXVI, 
Fig. 45, v). In the centre of the tissue of the tubercle procambium- 
elements are visible, which branch and obviously lead to the embryonic 
spines (PL XXVI, Fig. 45, v). One of the small conical outgrowths of the 
tubercles may here be described more in detail. It is covered by an 
epidermis, which is continuous with the epidermis of the neighbouring 
embryonic spines and the epidermis of the whole tubercle (PL XXVI, 
Fig. 45, e). The tissue inside the young spine may be seen to be connected 
with the procambial strands forming lower down in the tubercle. From 
the epidermis which covers the tissues of the cushion lying between the 
spines, arise hairs, which form an important part of the spine-cushion 
in some of the later stages of its development (Pl. XXVI, Fig. 45, i). 
At this stage the outer cells of the sides of the cylindrical body of the 
tubercle are beginning to show the regular arrangement into palisade-rows. 
The depression immediately outside the spines now gradually becomes 
more accentuated by a ring-like outgrowth, which grows up all round 
the cushion and finally surrounds the spines so that the spines come 
to stand in a cuplike depression (PL XXVI, Fig. 43). 
The spaces between the spines and between the younger tubercles 
in different stages of development are filled with hairs developed from 
the cushion of spines (PL XXVI, Fig. 39). Gradually the spines elongate, 
but they still at first stand erect on the tubercle and appear quite colour- 
less. Later on they assume a reddish colour. 
The procambium in the wart gradually becomes vascular tissue, but 
the lower cells of the spines, although they may for some time be connected 
with the procambial tissue, do not become vascular (PL XXVI, Fig. 44, Ji). 
Later on, the tissues at the lower end of the spine give rise to the hard, 
air-containing fibres. The epidermis of the spine is never thrown off, 
and the mature spine therefore remains covered with its original epidermis. 
The hairs found so plentifully between the mature spines also are epidermal 
structures. 
The spines develop in such a way that in the earlier stages the larger 
ones are on that part of the cushion which is nearest the organic centre of 
the main axis. They may thus at one end be 0-4 mm. long and at the 
other end i*a mm. At a later stage they may be 1*5 mm. at one, and 
