CHAP, xxx TRANSPIRATION IN LAND-PLANTS 107 



intercellular spaces are larger in shade-leaves (sciophylls) than in sun- 

 leaves (heliophylls), better developed in moist than dry air. But for 

 reasons already given it is generally true that in xerophytes growing on 

 land the air-containing intercellular spaces are very narrow ; in this 

 respect Altenkirch's x measurements of respiratory cavities may be 

 mentioned. But exceptions occur ; for instance, in Restiaceae there are 

 wide air-spaces, which possibly play a part in the assimilation of carbon 

 dioxide, in additi6n to very narrow ' girdle-canals '. 



' Girdle-canals ' also occur in the Australian desert -plant Hakea 

 suaveolens, as well as in Olea europaea, Kingia, 2 and in some arenicolous 

 grasses such as Festuca rubra and Triticum acutum. 3 They are narrow 

 intercellular spaces running round the palisade cells parallel to the leaf- 

 surface ; by this tortuous course the escape of water-vapour is rendered 

 more difficult. Certain desert-plants, such as Cynodon Dactylon, and 

 Sporobolus spicatus, have a maze of extremely fine meandering inter- 

 cellular canals, 4 but it is not certain that these various forms of intercellular 

 spaces are designed to depress transpiration. 



C. Chlorenchyma. 



It is characteristic of land-plants as opposed to submerged water- 

 plants to possess dorsi- ventral leaves, and in particular palisade tissue. 

 The latter is greatly developed in xerophytes by an increase in either 

 the number of layers or in the height of the cells, or by both means. It 

 has already been mentioned 5 that there is a difference of opinion as to 

 the significance and cause of this structural feature, and the suggestion 

 has been made that it is most closely correlated with dryness of the 

 atmosphere and with transpiration. Light doubtless also plays a part 

 in the matter, for the oblique orientation of palisade cells must be due 

 to illumination. 6 In halophytes growing on land the height of the palisade 

 tissue is increased by the salts contained in the soil, as has been proved 

 by Lesage. 



D. Other Means of Regulating Transpiration. 



Ethereal oils occur especially in xerophytes ; the garigues and maquis 

 of Mediterranean countries, 7 and the campos of Brazil are scented with 

 Cistus, Labiatae, Verbenaceae, Compositae, and Myrtaceae, just as are 

 European downs with wild thyme, and Asiatic steppes with Artemisia. 

 Neither the origin nor the significance of the correlation between dryness 

 of climate or of soil and the occurrence of ethereal oil has yet been 

 explained. These oils evaporate more readily than water, and surround 

 the plant with aromatic air. According to Tyndall, air rich in ethereal 

 oil is less diathermanous, that is to say, permits the passage of radiant 

 heat to a less extent than does pure air ; according to this view, ethereal 

 oils diminish insolation and consequently transpiration. 8 



It is possible that ethereal oils are of utility in other directions ; they 



1 Altenkirch, 1894. 2 According to Tschirch. 



1 According to Giltay, 1886. 4 Volkens, 1887. 5 See p. 21. 



8 See Warming, 1897. ' See Beck von Mannagetta, 1901, and others. 



* Volkens, loc. cit., and others. 



