H2 ADAPTATIONS. OECOLOGICAL CLASSIFICATION SECT, m 



slender, and often copiously branched ; the leaves of some species, such as 

 Genista tinctoria and Spartium junceum, are of relatively considerable 

 size, but fall off early when once they have performed their assimilatory 

 functions ; but the leaves of other species are from the outset very 

 reduced in form and function. The stem is terete, or deeply furrowed 

 with stomata and palisade in the furrows and mechanical tissue in the 

 ridges. This form of shoot is very common in Mediterranean Leguminosae, 

 particularly in Genista, Retama, Cytisus, and Genisteae in general, also 

 in Casuarina, Ephedra, a number of Chenopodiaceae, for example in 

 Anabasis (which, however, is mainly halophytic), in Capparis aphylla, 

 Periploca aphylla, and Polygonum equisetiforme. 1 



3. The juncoid shoot, as represented by many species of Juncus 

 and Cyperaceae, is tall, terete, aphyllous, and unbranched, being in 

 form similar to the leaf of some of the same species. The relative 

 proportions of surface and volume in such a shoot have already been 

 explained. This form of shoot also occurs in numerous marsh-plants, 

 such as Scirpus lacustris and S. palustris, Junci genuini, and others 

 belonging to the same families. 2 The Restiaceae also include shoots 

 belonging to this type. 



4. Acicular cladodes of Asparagus. 



5. The flattened shoot is an aphyllous form which is upright, or exposes 

 its profile : as examples may be cited Muehlenbeckia platyclada, Phyllo- 

 cladus, and Carmichaelia australis. The stem in some cases (Ruscus, 

 Semele) is so leaf-like that it is preferably included under the category 

 of sclerophylls, a course that has been adopted above. 



6. The spinose shoot of Colletia and others. 



7. The salicornioid shoot, as represented by Salicornia, Arthrocnemum, 

 and other Chenopodiaceae. 



8. The cactiform shoot is met with under various forms in Cactaceae, 

 Euphorbia, and Stapelia. It will be referred to subsequently in connexion 

 with succulent-stemmed plants. 3 



III. REGULATION OF ILLUMINATION 



As light has a heating effect upon the plant and thus promotes tran- 

 spiration, and as intense light is injurious to chlorophyll, many land-plants 

 possess devices by the aid of which the assimilating organs avoid too in- 

 tense illumination. These devices are temporary or permanent in nature : 



A. Movements by which Illumination is regulated. 



Many plants have an extremely delicate power of appreciating the 

 intensity of light, and can regulate the amount falling upon them by 

 movements of leaflets, which place their blades at definite angles to the 

 incident rays, the intensity of which determines the precise angle. When 

 the light is moderate, as in the early morning, the blades are exposed 

 as fully as possible to the light, whose rays strike them approximately 

 at right angles. But when the light becomes stronger the leaves more 

 and more assume a profile-lie, so that the angle of incidence becomes 



1 See the cited works of Pick, 1881 ; Volkens, 1887 ; Schube, 1885 ; Ross, 

 1887; Nilsson, 1887; Kerner, 1887; Schimper, 1808. 



2 See Chaps. XLIII, XLV. See p 123 



