CHARACTERISTIC FORMS OF LEAVES AND SHOOTS. 



2iy 



are characterised by the preponderance of the axial mass with a very small amount 

 of leaves, the latter, on the contrary, by the preponderance of leaves closely packed 

 round a short stem. If the lower parts of a plant produce slender lateral shoots 

 with small scales growing upon or beneath the earth, and after rooting at a con- 

 siderable distance from the mother-stock produce foliage-shoots or shoots stronger 

 than themselves, they are called stolons, as, for instance, in jEgopodtum Podagraria, 

 Fragarm, Siruihiopteris germanica, and in Mnium and Caiharinea among Mosses. 



The greatest degree of variation from the ordinary forms of shoots is displayed 

 by the flat leaf-like axes and branch-systems, and by the stem-tendrils and spiny 

 shoots which occur frequently in Angiosperms. Leaf-like axes {phylloclades) 

 are found in those Phanerogams in which large green foliage-leaves are wanting, 

 and they replace them physiologically; their axial structure is of considerable 

 superficial extent, and they produce and expose to the light large quantities of 

 chlorophyll; they generally bear only very small membranous scale-leaves. Ex- 

 amples may be found in Phyllocladus among Conifers, Ruscus among Mono- 

 cotyledons, and among Dicotyledons in Muhlenbeckia platyclada (Polygonaceae), 

 ' Xylophylla (Euphorbiacese), Carmichaelia (Papilionacese), and in Opuntia hrasiliensis 

 and Rhipsalis crispaia (Cactaceae), &c. 



Stem-tendrils, like leaf-tendrils, are long, slender, filiform structures, which have 

 the power of winding spirally round slender bodies in a horizontal or oblique position 

 with which they come laterally into contact, and thus serve as climbing organs ; they 

 spring laterally from shoots which have not the form of tendrils, and are distinguished 

 by the absence of foliage-leaves, their power of forming leaves being mostly limited 

 to very minute membranous scales. They are usually easily distinguished from leaf- 

 tendrils by their origin, position, and by the production of leaves ; cases, however, 

 occur where the morphological nature of a tendril is doubtful, as, for instance, 

 in Cucurbitacese ^ Evident examples of stem-tendrils are to be met with in 

 Vilis, Ampelopsis, and Passiflora. Shoots which bear strongly developed foliage- 

 leaves on long slender internodes, and which have the power of winding in an 

 ascending manner round upright supports, are not considered tendrils, but are 

 called twining or climbing stems'^; a distinction is drawn between tendril- 

 climbers (as Vitis) and stem-climbers (as Phaseolus, Humulus, Convolvulus, &c.). In 

 Cuscuta, where the primary shoot and all the lateral shoots, except the inflorescences, 

 twine in the manner of tendrils and of climbing stems, and where foliage -leaves are 

 also entirely suppressed, the pecuHarities of tendrils and of climbing stems are to a 

 certain extent united. A distinction similar to that between stem-tendrils and climb- 

 ing stems is also possible in leaves; the foliage-leaves of Lygodium exhibit con- 

 tinuous growth in length, and behave completely like climbing stems, the rachis 

 of the leaf corresponding to a climbing axis, and the leaflets to its foliage-leaves ^ 



The axial shoots of many Angiosperms have, like the leaves, the power of 



^ According to Warming these are also metamorphosed branches. 



^ Compare H. von Mohl, Ueber den Ban und das Wmden der Ranken und Schlingpflanzen. 

 Tubingen 1827. [See also Darwin, On the Movements and Habits of Climbing Plants, London 

 1875.] 



^ Compare Book II, Ferns, and Book III, on the Physiological Signification of Tendrils and 

 Climbing Stems. 



