ii6 ADAPTATIONS. OECOLOGICAL CLASSIFICATION sect, iii 



membranous stipules of species of Paronychia, Herniaria, and other 

 plants, clothe young parts of the shoot with a dense silvery investment. 



Old leaves and remnants of leaves in many cases act in the same manner. 

 ' Tunic-grasses ' is the term employed by Hackel ^ to designate those 

 grasses in which the lower parts of the leaves remain attached long after 

 their upper parts have died, persisting either as coherent, firmly closed 

 sheaths, or in a macerated condition. These tunics depress transpiration, 

 and store water ; they occur in grasses growing on dune, steppe, or 

 desert, for example, in Nardus stricta, Andropogon villosus, Scirpus 

 paradoxus, S. Warmingii, species of Aristida.^ A similar relationship 

 exists in the Velloziaceae living on the mountain-tops and high plateaux 

 of Brazil.^ In certain South African species of Oxalis the bulbs are 

 invested by peculiarly constructed leaves ^ ; the dead bulb-scales of 

 Tulipa praecox bear a dense felt of hairs. Here we may mention the 

 compact clumps, such as those of the ' cushion-plants ' Raoulia and 

 Azorella, consisting of closely-packed shoots and remains of shoots ; 

 they are met with in subglacial vegetation, and especially in South 

 America, and are often so hard that it is difficult to cut or break them ; 

 in this case one shoot protects another the old leaves protect the 

 young. 



Many other methods are adopted to protect the youngest part of 

 the stem and leaves, and are described in the papers cited .^ 



The roots of many epiphytes, including the asclepiadaceous Concho- 

 phyUum imbricatum, are screened from excessive transpiration by leaves, 

 which cover them closely, and keep them surrounded with moist air.^ 

 The roots of some grasses living in the Egyptian desert, for instance, 

 species of Aristida, Andropogon, Elionurus, Panicum, and Sporobolus, 

 are surrounded throughout their length by a sheath of sand, the grains 

 of which are glued together by an adhesive substance excreted by the 

 root-hairs. To a less marked extent the. same is true of dune-grasses 

 in northern Europe." Volkens ^ interprets this as a device for checking 

 evaporation. 



V. THE ABLATION OF RAIN-WATER 



It is of importance to the land-plant that its leaves shall not remain 

 too long wet with rain-water ; it is necessary for their surfaces to dry 

 quickly, if transpiration is to be resumed.^ And there seem to be adapta- 

 tions serving to carry rain-water rapidly away. It is in the tropical 

 rain-forest that such devices are especially met with, though they are 

 perhaps not entirely lacking in temperate countries. Jungner, working 

 in the rainy Kamerun, and subsequently Stahl in Java, arrived at essen- 

 tially the same conclusions. They regard as adaptations : 



1. A smooth cuticle that cannot be wet ; this device is very wide- 

 spread. 



2. Drip-tip, as Stahl terms the long leaf -tip terminating a blade 

 that often becomes suddenly narrow ; it is typically represented in . 



^ Hackel, 1890. 



^ Hackel, 1890. See also illustrations in Warming, 1892. Henslow, 1894. 



^ Warming, 1893. ^ Hildebrand, 1884. 



^ Also see Lubbock, 1899. * Gobel, 1899. ' Warming, 1907-9. 



* Volkens, 1887. * Concerning ombrophobous and ombrophilous plants, see p. 3 2. 



