408 Darbishire. — Observations on Mamillaria elongata. 
This function is accentuated by the presence of a colourless hypoderma 
in a plant like Ilex aquifolium. Paraheliode structures may have two or 
one of two functions. They may damp the strong light in order to protect 
the chlorophyll, or they may do so for the purpose of keeping down the 
rate of transpiration. In the latter case only would they be xerophil 
structures. 
Examples of paraheliode structures could be added to in large 
numbers. I would like, however, without citing too many examples, only 
just to quote a few, in order to make it clear in which way the word 
paraheliode should be used, if it is adopted. 
A paraheliode is an organ for damping the effect of the strong sunlight. 
It thus acts like a parasol or sunshade. A layer of cells, like the hypo- 
derma, can well be called a paraheliode. The whole set of spines crowning 
the tubercles in Mamillaria form a paraheliode. I can well imagine that 
the masses of hard white bast-plates, which we so frequently meet with 
in many plants of dry and light regions, are really, in part at least, para- 
heliodes to protect the underlying tissues. But I am in this case merely 
making a suggestion as to a possible explanation. I am thinking here 
of some of the leaves, sections of which are figured by Volkens in his 
splendid book on the Egyptian desert-flora (33, Pis. XVI, XVII, XVIII). 
The green tissues not unfrequently have interposed between them and the 
direct sunlight thick plates of strong mechanical tissue, which absorb a very 
large amount of light. This tissue is no doubt of most importance when 
these plants are in a dry condition. The leaves then roll up, and the green 
cells would thus be in almost complete darkness. The leaves of Aristida 
ciliata would seem to be of this nature (33, PI. XVI, Fig. 4). From the 
description by Volkens they appear to be permanently rolled up longi- 
tudinally (33, p. 150). 
The red colour clearly offers in many cases a protection to the 
underlying chlorophyll or protoplasm against the undue strength of the 
sunlight (36, p. 18). 
C. Concluding Remarks. 
Before putting together in an abbreviated form the results of this 
investigation, I would like to make some general concluding remarks. 
I have not so far referred to the function which is very generally 
assigned to many of the spines, thorns, and prickles of many plants. 
Large and strong prickly structures are generally credited with being 
defensive organs for keeping off animals, which might otherwise graze on 
the plants concerned, and thus destroy or at least injure them. 
With regard to the spines of the Cactaceae this is the view held by 
Goebel (12, p. 35), and, I think I may say, by botanists generally. Ganong 
