BY SELF-ADAPTATION TO THE ENVIRONMENT. 247 
In some cases the inner sheath is colourless and acts as a 
water-storage tissue (Oligomeris subulata). It is,I think, really 
homologous with the endoderm. 
M. Dufour * found that chlorophyllous tissue is much more 
developed in sunlight than in shade; and it may be added that 
a similar phenomenon occurs in Alpine plants and plants of high 
latitudes as compared with the same species growing at low 
altitudes and latitudes. In these cases the deeper green tint 
is also due to the uninterrupted sunlight. Hence it is not sur- 
prising to find the chlorophyll-tissues reach a high development 
under the intense light in the desert, whether from the sun 
direct, or reflected upwards from the sand. 
X. Ligneous Tissues. 
Messrs. Dr. D. H. Scott and G. Brebner have described f the 
histology of Acantholimon glumaceum (Plumbaginez), and Prof. 
D. Oliver had previously examined { that of Acanthophyllum as 
well (Caryophylleew). These are both desert plants ; and I might 
now add a great many more anomalous and subanomalous stems 
of plants growing in the North-African deserts, some of which 
Dr. G. Volkens has described and figured §. The general con- 
clusion deducible from a study of their peculiarities is that these 
are due to the climatal conditions under which they grow. 
Summing them up, they may be enumerated as follows :—There 
is a general tendency to lignification; with an absence of me- 
dullary rays (Zilla myagroides, Bassia muricata); if they be 
present, they are comparatively few and have thick walls (Farsetia 
africana, Helianthemum kahiricum, Ochradenus baceata); the 
fibro-vascular cords may form “islands,” as seen in a trans- 
verse section, imbedded in dense tissue (Statice pruinosa, Atri- 
plex leucoclada, Pityranthus tortuosus) ; the “ wedges” of wood 
may fail to form a regular zone, but be more or less isolated 
and imbedded in water-storage tissue (Zamarix mannifera, 
Anabasis articulata). The wood may be deeply indented with 
cortical invasions, as Prof. Oliver has shown to be the case in 
Acanthophyllum. Dr. Scott observes that this is due to the fact 
* Ann. des Sci. Nat., Bot. sér. 7, tom. v. p. 311. 
t Ann. of Bot. vol. v. p. 259. 
¢ Trans. Linn. Soe. vol. xxii. p. 289. 
§ Die Flora der aegyptisch-arabischen Wiiste, &c. 
