i 2 6 ADAPTATIONS. OECOLOGICAL CLASSIFICATION SECT, in 



have long, tubular, branched receptacles. But there are still other kinds 

 of water-reservoirs. Certain plants contain, scattered about in their 

 general chlorenchyma, solitary or grouped cells which are larger than the 

 other cells, have thin walls and clear contents. Such occur in Nitraria 

 retusa, Salsola longifolia, Halogeton, Zygophyllum, and other plants 

 belonging to the Egypto-Arabian desert, 1 also in Barbacenia growing on 

 mountains in Brazil, 2 as well as in loranthaceous parasites. 3 In certain 

 cases it has been established that if a slice of a leaf be allowed to dry 

 these cells collapse, but expand again when water is supplied. 



Lignified idioblasts (tracheids*) with spiral or reticulate thickenings 

 occur in numerous other species, and are usually scattered in the same 

 manner ; they resemble the cells of the velamen of orchids 5 and the 

 porous cells of Sphagnum, 6 being short, tolerably thick-walled, but not 

 perforated ; they fill with air when the contained water passes out. 

 They show two types of distribution, being either grouped at the ends 

 of vascular bundles or dissociated from these. The latter is the case 

 in the leaves of many tropical orchids, 7 species of Crinum, 8 Nepenthes, 9 

 Sansevieria, Capparis and Reaumuria, 10 Salicornia, u and Centaurea. 12 

 They occur in other xerophytes and halophytes close to the ends of 

 vascular bundles ; in this position, especially in desert-plants, they 

 assume the form of huge, irregular tracheids with slit-like or elongated 

 pits, and, as they stand above the delicate blind ends of vascular bundles 

 in the leaf, they are often difficult to distinguish from tracheids apper- 

 taining to the bundles : arrangements of this kind occur in species of 

 Capparis and Caryophyllaceae. 13 These water-storing tracheids seem 

 to play the same role as do wood-vessels in vascular bundles, since they 

 fill with water and give it up again without collapsing. 



The parenchyma-sheath surrounding vascular bundles may perhaps 

 function as a water-reservoir in some Egyptian desert-plants 14 and in 

 Restiaceae. 15 



Translocation of water. Meschajeff 16 seems to have been the first to 

 point out that the older leaves of succulent and semi-succulent plants 

 often serve as water-reservoirs for the benefit of younger leaves ; for in 

 times of prolonged drought water is transferred to the younger parts of 

 the shoot from the older leaves, which shrivel and die. 



1 Volkens, 1887. 2 Warming, 1893. 3 Marktanner-Turneretscher, 1885. 



' Reservoirs vasiformes ' of Vesque, 1882 ; ' spiral cells ', ' storage-tracheids ' 

 of Heinricher, 1885. B See p. 104. 



See Chap. XLIX. ' Trecul, 1855 ; Kriiger, 1883. 



8 Trecul, 1855 ; Lagerheim, 1892. ' Kny u. Zimmerman, 1885. 



10 Vesque, 1882 a, b. u Duval-Jouve, 1868. " Heinricher, 1885. 



13 See Vesque, 1882 a and b ; Heinricher, 1885 ; Kohl, 1886 ; Volkens, 1887 ; 

 Schimper, 1898 ; Haberlandt, 1904. 



14 Volkens, 1887. 5 Gilg, 1891. 

 14 Meschajeff, 1883; see Burgerstein, 1904, p. 228. 



