252 REV. G. HENSLOW ON THE ORIGIN OF PLANT-STRUCTURES 
whilst the outermost series become almost woody in texture, as a 
protection against the hot sand in which they lie. 
The cortex of roots acts as a storage-tissue in many plants, as 
in Gypsophila Rokejeka, and grasses, such as species of Dan- 
thonia, while the axis in both is densely woody. 
A cause may be suggested for the development of the hyper- 
trophied condition of the cortex and medulla of roots, which act 
as water-storage tissues, in the excessive heat which the sand 
surrounding the roots acquires from the sun. In the hottest 
months and hours of the day, the temperature may rise to about 
130° F. (Volkens). 
Now M. E. Prillieux* has shown experimentally how an 
abnormal excess of heat in the soil affects the roots of beans, &c., 
in a precisely analogous manner, by enlarging the cells of the 
cortex and pith. While, however, in this latter case the disten- 
tion of the parenchymatous tissue is, of course, abnormal and 
pathological, in the desert plants it has become a characteristic 
important and hereditary feature. 
M. Battandier also calls attention to the fact that there are 
plants in which certain buds swell into tubercles capable of 
enduring the dry season, while the rest of the plant perishes; 
such are Saxifraga globulifera and Sedum amplexicaule. In 
Malta there are several species of Ranunculus, such as R. bullatus, 
which produce “root-tubercles,” which are thus enabled to 
survive the parching summer and can live therefore on the most 
exposed surfaces round the fortifications of Valetta. Romulea 
Bulbocodium and Iris Sisyrhynchium are similarly enabled to live 
and abound in barren rocky grounds of Malta. This island has 
also fifteen species of Allium, nine of Ophrys, and eight of Orchis, 
as well as many other bulbous plants, which can thus survive 
the intense summer heat to which they are subjected; annuals 
being in a decided minority except as weeds of cultivation. 
From M. Hackel’s observations on the tuberous processes 
in grasses which are formed in dry soil and disappear elsewhere, 
we, at least, have a strong suspicion, if nothing more, that 
all such structures are the outcome of the environment itself. 
This is also the conclusion of Mr. Scott Elliot, who notices how 
numerous are bulbous and tuberous plants in the Karoo of 
S. Africa. He says :—‘ Many orders have developed bulbs which 
- * “Altérations produites dans les plantes par la culture dans un Sol sur- 
chauffé,” Ann. des Sci. Nat. sér. 6, t. x. 1880, p. 347. 
