Dar bishire. — Observations on Mamillaria elongata . 379 
with the soil are generally protected by cork. The epidermal layer does 
not, however, seem to be thrown off till fairly late, if it is got rid of at all. 
It can in fact generally be distinguished even outside the deepest masses of 
cork by the sinuate radial walls of its cells. The phellogen takes its origin 
in the layer of cells immediately inside the epidermis. The cork-cells are 
frequently very much flattened and stretched. They may thus attain 
a length of 100 ju, being at the same time 20 \k and less in radial diameter 
(PL XXVI, Fig. 27). 
The main axis of the plant is traversed by a ring of bundles, which in 
my plants at least remained separate, fairly large medullary rays connecting 
the pith with the cortex (PI. XXV, Fig. 2). At any given point we can 
find between eight and twelve or fourteen bundles in transverse section. 
An examination of their structure shows the arrangement already 
described for the Cactaceae by several authors since the time of Schleiden 
( 25 , pp. 20-36). Solereder gives a brief summary of the literature which 
refers to the structure of the Cactaceae ( 28 , pp. 459-468). 
I will, however, briefly recapitulate the structure of our little plant 
(PI. XXVI, Figs. 21-27). 
The bast forms a small and inconspicuous part of the whole collateral 
bundle (PL XXVI, P"igs. 21, 22). Following the nomenclature of Vochting 
( 32 , p. 409) we can distinguish between large clear cambiform cells (/), and 
smaller and darker protophloem elements ( 0 ). The former measure 35 to 
40 by 15 to 20 n , in transverse view, being generally somewhat compressed 
radially ; the latter are more isodiametrical, measuring about 3-5 to 7 //. 
across. In length both kinds of bast-cells measure about 90-100 In the 
same way as we shall be able to notice in the xylem later on, the transverse 
walls of the bast elements are all found to be almost at the same height. 
Both kinds of cells finish longitudinally at the same level, their ends being 
but slightly drawn out. We might almost distinguish nodes and internodes 
in the bast, as later on in the wood. The protophloem-cells form groups of 
two to three to twelve or more cells, which lie embedded in the large 
cambiform cells. Both contain protoplasm, the protophloem-cells, however, 
more abundantly. 
The bundles possess but little cambium, which very slowly adds on 
new tissue (r). This consists only to a very small extent of bast, being 
chiefly wood. 
We now come to the wood, the elements of which occur in four 
different forms, three of which only are represented in every central bundle 
of the main stem. 
The protoxylem is made up of the long and narrow spiral tracheids 
(Pl. XXVI, Fig. 21,/) so generally met with in this part of the bundle. 
They vary in diameter from 10 to about 20 /x, at which size, however, they 
are already passing into the metaxylem. The thickening forms a fairly 
