250 REV. G. HENSLOW ON THE ORIGIN OF PLANT-STRUCTURES 
7 
called because the water-cells are so conspicuous over the surface 
of the leaf. Many other plants are provided with them, but they 
are not so conspicuous to the naked eye; thus species of Reseda 
(as R. arabica), Gypsophila Rokejeka, Malcolmia egyptiaca, Helio- 
tropium arborenm, Hyoscyamus muticus *, &c. Atriplex leuco- 
clada has globular water-cells on slender supports; in this, 
asin Atriplex Halimus, the hairs finally collapse, and by sticking 
together form a sort of protecting sheath all over the surface of 
the leaf. 
In the interior of the leaf the chlorophyll-tissue is sometimes 
so abundant that the leaf is perfectly green throughout ; in other 
cases the central tissue is nearly or quite devoid of chlorophyll- 
grains and acts as a storage-tissue. In Atriplex Halimus the 
outer layer of palisade-cells is clear and utilized as storage ; while 
in some, as Nitraria retusa, large isolated storage-cells lie im- 
bedded within the palisadic tissue, some others of the elongated 
cells being reservoirs of tannin. 
Of the above varieties the central storage-tissue is the most 
important and is well seen, as it occupies by far the greater space, 
in such succulent leaves as those of Mesembryanthemum and Aloé. 
Sign. Arcangeli has lately observed that Atriplex nummularia 
possesses below the epidermis of the leaves an uncoloured paren- 
chyma which covers the assimilative layer and represents a tissue 
of water-storage +. 
In stems the cortex and medulla act as storage-tissues, the 
former gradually passing into cork, which of course is a great 
protective tissue in many desert plants. 
Of subterranean structures the bulb-scales of species of 
Allium, Pancratium, &c., and the cortical region of roots of 
grasses, may act as storage-organs. 
In some exogens the roots and subterranean stems assume 
special forms which act as reservoirs. Thus, in the genus 
Lrodium there are three desert species which develop tuberous 
structures on the roots, which Dr. Volkens proved to be water- 
and not starch-storing tissues. This observation has been 
* The epidermal water-cells give a pale green colour to the leaf, and render 
it difficult to dry completely, except after many days in the press; perhaps 
because the water in all the plants is more or less thickened by gummy or other 
matter, 
+ “Sulla struttura delle foglie dell’ 4, nwmmularia, Lindl., in relazione alla 
assimilazione ” (Nuovo Giorn. Bot. Ital, vol, xxii. p. 426). 
