404 Dar bishire. — Observations on Mamillaria elongata. 
Being not very closely set, the spines do not interfere with the light 
which the plant needs to be supplied with for the photosynthetical 
functions. But they will probably act as a useful sunshade also in this 
direction. 
The whole set of spines again serves as a protection for the main ending 
of the medullary bundles in the tubercle. These with their storage-water 
are protected from the strong light by the broad lower ends of the spines, 
glistening with the imprisoned air. A mass of white glistening hairs 
between the bases of the spines helps in the same way. No water can 
escape by evaporation at this end, because a broad plate of corky tissue 
underlies the set of spines, and almost overlies the mass of storage 
tracheids. How effectually the spines do keep off the light may be seen 
from the light colour of the cells which lie inside the storage-tracheids. 
They contain hardly any chlorophyll. 
The single central spine acts in the same way as the other spines, 
but is most effective when the sun shines directly on to the tip of the 
tubercle. 
It may be mentioned here that the apical and more delicate portion 
of the whole plant is extremely well protected against the strong light 
by the spines and hairs arising from the young developing tubercles 
(PI. XXV, Fig. 1 8). These are at first very closely set, and completely 
obscure the growing apex. The function which the whole set of spines 
performs for the benefit of the plant is, therefore, to sum up, that of 
a screen or sunshade. 
I consider this function so important that I have thought it worth 
while calling such an organ as the whole set of spines represents 
a paraheliode . 
Attention was already called in 1876 by Wiesner to the possibility of 
hairs being of use to the plant in acting as a protective screen between 
the strong sunlight and the chlorophyll of young developing and therefore 
rather delicate organs of the plant. Wiesner instances the case of 
Tussilago Far far a. The coat of white hairs on the upper surface remains 
on the leaf as long as the green colour has not fully developed. It is then 
thrown off. If removed prematurely the formation of the chlorophyll 
seems to be impeded (39, pp. 24, 42). 
Warming also refers to this function of hairs, as damping the effect 
of the sun’s rays (36, p. 18). 
I do not intend in this paper to refer to the question of the function 
or meaning of the deposits of calcium oxalate, nor to the well-known strong 
acidity of the cactaceous cell-sap (vide 3 and 34, &c.). 
