Dar bishire. — Observations on Mamillaria elongata. 399 
The cortical bundles pass along just inside the palisade-cells 
of the tubercle and then end blindly, before the tip of the latter is reached, 
without changing very much in structure with the exception of the 
bast, which is absent from the endings of the cortical bundles (PI. XXV, 
Figs. 7, 8, 15). 
The medullary bundles at first anastomose freely, and finally form a 
cup-like mass of wood-elements just beneath the tip of the tubercle (PI. XXV, 
Figs. 10 to 15). At this point the bast again is seen to be absent, although 
it was present in the bundles lower down (PI. XXV, Figs. 4 to 6). The 
wood-elements also have undergone a remarkable change. The long, thin 
spiral tracheids have gradually given way almost entirely to a mass of large, 
broad and stout cells with spiral and reticulate to annular thickening. 
The cup-like ending of the medullary bundle-system consists almost 
entirely of these cells, especially towards its outer margin, where it ends 
blindly (PI. XXV, Figs. 15, 19, 20). These cells are surrounded imme- 
diately by large and clear cells, which by means of their large vacuole, no 
doubt, exert a strong osmotic suction on the water contained in the 
neighbouring tracheidal elements. 
Not only are the surrounding parenchymatous cells large and more or 
less round in form, but the separate wood-cells also offer more than one 
flat surface to be acted upon by these cells. I have no doubt that the 
large wood-cells act also as storage-tracheids. 
The water, with its solutes of raw food material, is drawn into the 
surrounding parenchymatous cells. In due time the water in the form of 
vapour passes out of those cells which are bordered by extensive inter- 
cellular spaces, into the latter, and finally makes good its escape through 
the stomata, by which the epidermis of the tubercle is interrupted. 
We have thus far followed out the path of the transpiration-stream. 
It might be of interest just to recall in a tabular form the measure- 
ments of the tracheids in the different parts of the plant, with reference to 
their varying function. 
Nature of 
thickening. 
Breadth. 
Length. 
Root protoxylem 
Annular 
IO-I5 fJL 
metaxylem 
Annular 
10-2 6 1 a 
200 [X 
Stem protoxylem 
Spiral 
10-20 ft 
IOO [X 
metaxylem 
» 
3°~ 4° 
IOO ix 
Bundles leading to tubercles 
}> 
12-16 IX 
Tubercles Medullary 
53 
8-12 ix 
Endings 
Cortical 
)> 
16-20 [X 
Medullary 
)) 
20-60 [X 
100-200 [X 
