298 



MAINTENANCE OF A FREE PASSAGE FOE AQUEOUS VAPOUR. 



aimed at is to keep a free passage for the water- vapour which must escape from the 

 stomata. To bring this about, the stomata are situated in grooves filled with air 

 which are sunk in the green tissue, and which give a striped appearance to the 

 branches. Water cannot force out the air from these narrow furrows which run 

 along the green branches and twigs, six of them to each branch. The branches may 

 remain submerged in water for an hour without a trace of moisture entering the 

 furrow. Moreover hairs are present in the furrows as a guard against moisture. 

 These cannot be wetted, and the air adheres to them just as to the cuticular pegs of 

 the bamboo leaf. A clear idea of this arrangement is given in the transverse 

 section of the stem shown in fig. 69 3 and 69 4 . The adjacent section of the green 

 branch of the Australian Casuarina quadrivalvis shows that these curious plants 

 also have exactly the same arrangements, that the stomata He at the bottom of 



^COQQ^&pOCT 



Fig. 68. — Stomata in Pit-like Depressions. 



i Surface view of a leaf of Dryandra Jloribunda. A portion of the hairs which fill the pit is removed, in order to show 

 thestomata; x 350. 2 Vertical section through a leaf of Dryandra Jloribunda ; x 300. 



narrow furrows which run along the green leafless branches, and that peculiar hair- 

 structures are present in the" furrows, to which the air adheres, forming a barrier 

 against water, exactly as in those of the Gytisus. The Casuarinse, which must 

 finish their work for the year during the very short rainy period of their native 

 country, require during this time arrangements providing for unhindered transpira- 

 tion no less than does the Gytisus in the Southern Alps. Altogether this con- 

 trivance is found to be present in only a limited number of cases; in perhaps only 

 twenty papilionaceous shrubs, most of which belong to the Spanish flora, of the 

 genera Reta/ma, Genista, Ulex, and Sarrothamnus, in addition to the Australian 

 Casuarinas, and in allied species of Gytisus (holopetalus, purgans, ephedroides, 

 equisetiformis, candicans, albus, &c). Most remarkably also this arrangement 

 occurs in a small species of Broom (Genista pilosa), which is distributed over the 

 mountains of Central Europe, over the heaths of the Baltic Lowlands, Denmark, 

 Belgium, and England. And the presence of this contrivance here is the more 

 strange, from the fact that the green branches with their furrows, in which lie 

 stomata, are not leafless, but, on the contrary, are provided with a comparatively 

 well-developed foliage. 



Among the most peculiar plants whose stomata are concealed in hidden nooks, 



