io8 ADAPTATIONS. OECOLOGICAL CLASSIFICATION sect, hi 



may, for instance, protect plants against herbivorous animals, as Stahl ^ 

 suggests. Detto - doubts the correctness of the view that ethereal oils 

 serve as a screen against transpiration, and takes the same view as Stahl. 



The significance of latex is not definitely established. According to 

 Haberlandt, Schullerus, Pirotta, and others, laticiferous tubes are con- 

 ducting channels, but according to Kerner they form a defence against 

 animals at least in Cichoriaceae.^ 



By the various agencies described above, the transpiration of leaves 

 is brought into harmony with the different environments. But it must 

 not be concluded that xerophytic leaf-structure is inconsistent with 

 capacity for vigorous transpiration : indeed Bergen * found that the 

 absolute amount of transpiration (the amount of water lost in a unit of 

 time) was scarcely less in the sun-leaves (heliophylls) of certain ever- 

 greens, including Olea europaea and Quercus Ilex, than in Ulmus cam- 

 pestris and Pisum sativum. 



II. DIMINUTION OF THE EVAPORATING SURFACE 



The extent of the transpiring surface plays an important part in 

 determining the amount of transpiration : other relations being constant, 

 the larger the surface the greater the transpiration. As foliage-leaves 

 are essentially the organs of transpiration, it is their size and number 

 which regulate this function and which therefore vary in the different 

 species in accordance with climatic conditions. Divers means adopted 

 to depress transpiration are treated in the succeeding paragraphs. 



A. Temporary Diminution of Surface. 



The most decisive method by which a plant can diminish its tran- 

 spiring surface is the shedding of all strongly transpiring parts before the 

 commencement of the dry season. This takes place, first, in all annuals 

 which die after the seed has ripened : all seeds are very efficiently pro- 

 tected from desiccation. In harmony with this is the very high per- 

 centage of ephemeral species in deserts and similar places ; within the 

 short rainy season, sometimes only from one to two months in length, 

 these plants complete their whole life-cycle, germinating, flowering, setting 

 seed, and dying, so that they pass through the dry season in the form of 

 embryos enclosed in seeds ; Odontospermum (Asteriscus) pygmaeum, the 

 'Rose of Jericho ', is such a plant.^ 



Similar behaviour characterizes all bulbous and tuberous plants, as well 

 as other ' renascent ' herbs whose subterranean shoots serve as reservoirs 

 of food and water during the dry season : the epigeous shoots, with their 

 extensive transpiring surfaces, are dispensed with while drought prevails, 

 and the latent vitality is confined to the underground shoots. When 

 moisture is once more supplied these species hasten to thrust new shoots 

 and flowers into the light. ^ In fact, the rapid onset of spring after the 

 first few showers of rain in deserts, steppes, and similar places, has often 

 been mentioned with surprise by travellers. 



In like manner behave woody plants which shed their foliage before 



* But see Burgerstein, 1904, pp. 133, 214. ^ Detto, 1903, 



^ See also p. 125. ' Bergen, 1Q04. ^ Volkens, 1878. 



' See p. 8. 



