222 REV. G. HENSLOW ON THE ORIGIN OF PLANT-STRUCTURES 
main due to a want of water *, which always prevents the for- 
mation of cellular tissue; while this deficiency of parenchyma is 
associated with a hardening of the fibro-vascular mechanical 
elements. The converse conditions are sometimes witnessed, e. g- 
in a plant of Zilla myagroides, which was the only species raised 
from seed out of many sown in the Botanic Garden of Cairo by 
Dr. E. Sickenberger. It not only bore well-developed leaves, 
but the spines, though formed through the forces of heredity, 
were very slender and subflaccid instead of being intensely 
rigid. 
As an interesting illustration of a highly spinescent plant 
belonging to the Cucurbitacew, an order in which it would be 
least expected, is the Narras plant of Caffraria (Acanthosicyos 
horrida). It grows on the sandy downs on the sea-coast. It 
has no leaves, but double spines studded all over the branches, 
forming impenetrable bushes which spread widely and attain the 
height of a man. It is curious that although the seeds germinate 
readily, all attempts to cultivate it in Europe have failed; just 
as Dr. Sickenberger and I failed with the numerous desert plants 
of Egypt, for, as stated, he was only successful with Zilla mya- 
groides. Another remarkably spinescent plant is Aciphylla 
squarrosa of the order Umbellifere, described by Sir J. D. 
Hooker in his ‘ New Zealand Flora,’ vol. i. p. 87. 
The hardening of the mechanical tissues generally, which so 
often results in special spiny processes, is brought about by 
drought and other conditions of the environment, and is one of 
the best means for resisting the intense heat of the desert. 
M. de Candolle ¢ called attention to this tact. He says :— 
‘‘Very hard wood resists heat because it encloses but little 
aqueous juices, so there is but little to evaporate.” Similarly he 
remarks upon the great value of cork for resisting both extreme 
* So Mr. Belt, in ‘The Naturalist in Nicaragua,’ says (p. 46) :—* This spiny 
character of vegetation seems to be characteristic of dry rocky places and tracts 
of country liable to great drought.” 
Similarly Grisebach, aescribing the flora of Kalahari, South Africa, and 
alluding to spinescent species of Acacia, says:—* Tous ils portent des épines 
qui, chez A. horrida, ont 5 4 8 centimétres de longueur. La division limitée du 
feuillage et l’exiguité des surfaces sont autant de traits en rapport avec le dé- 
veloppement des organes piquants et de la sécheresse du sol” (La Végét. du 
Globe, ii. p. 252). He also alludes to similar features of the Tibetan flora 
(i. p. 614). 
+ “Essai élémentaire de Géographie Botanique,” Dict. des Sci. Nat. vol. xviii. 
