224. REY. G. HENSLOW ON THE ORIGIN OF PLANT-STRUCTURES 
shoots growing in a saturated atmosphere. They reappear as 
soon as the same plants are allowed to grow in the ordinary way. 
It may be added that when the spines are arrested, the peculiar 
odour of O. repens is present, and the flowers also are larger 
and are like those characteristic of that subspecies *. 
M. Lotheliert carried out very similar experiments with other 
spinescent plants. He thus found that by growing Berberis 
vulgaris in a moist atmosphere it bore no spinescent leaves, as the 
parenchyma was well-formed between the ribs and veins; but 
in a perfectly arid atmosphere it bore spines only. Intensity of 
light also favoured their production. He found from a micro- 
scopical examination that in a section of a spine exposed to 
moist air the vessels of the xylem are few in number and the 
pericycle is not lignified. In a dry air the xylem forms a con- 
tinuous ligneous circle and the pericycle is also lignified. I have 
myself repeatedly corroborated this observation of the special 
consolidation of fibrous tissues in several instances in different 
species of desert plants. This consolidation of the mechanical 
elements may be perhaps explained as a secondary result of the 
relative abundance of the assimilative tissues ; the palisade-layers 
being often two, three, or even four in number, and equally on 
both sides of a leaf of a desert plant. While there is a great 
arrest in area of the parenchymatous tissue—due to the feeble 
water-supply—the organized products are mainly utilized in the 
lignification of the supportive tissues. 
Conversely, if plants be etiolated by being grown in the dark, 
while the parenchymatous tissues are relatively in excess, such as 
the pith and cortex, M. Rauwenhoff has shown that the mechanical 
tissues are greatly reduced, the assimilative tissues being quite 
incapable of any activity t. 
Another instance is supplied by our common Water-reed, 
* Such were the results of my own experiments in 1891-92. I raised plants 
from the seeds obtained from these plants in 1893, to see if the hereditary 
trait of producing spines becomes less pronounced in successive generations. 
In August the plants had only grown to about five inches in height in conse- 
quence of the drought; but up to the date of writing this (Aug. 15th) they had 
developed no spines. 
t Revue gén. de Bot. 1890, p. 276; Bull. Soc. Bot. de Fr. 1890, p. 176; 
Comptes Rendus, 1891, exii. p. 110. 
} “Sur Jes causes des Formes anormales des Plantes qui croissent dans 
Yobscurité,” Archives Néerlandaises des Sci. exac. et nat. t. xii. p. 297. 
