96 



EXTERNAL CONFORMATION OF PLANTS. 



o grow underground, and to send up only at intervals long foliage-leaves or shoots, 

 vhich subsequently disappear in their turn and are replaced by others. When 

 ;uch shoots or systems of shoots lie horizontally or obliquely on the ground, and 

 )roduce lateral roots, they are called Rhizomes (Fig. 148) (as in Iris, Polygonatum, 



l^^ 



Fig. 148. — Rhizome oi Pteris aqicilzna ; /, //, /// the underground creeping axes ; ss the apex of one of them ; i-6 the basal 

 parts of the leaf-stalks ; 7 a young leaf; da decayed leaf-stalk, the basal portion of which is still living and bears a bud HI a; the 

 hairy threads are roots which arise behind the growing apex of the stem. 



Pteris aquilina, and many other Ferns). Frequently they die at the posterior and 

 continue to grow at the anterior end. Underground tubers and bulbs are more 

 ransitory structures, usually lasting only for one period of vegetation ; the former 

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

 )f leaves, the latter, on the contrary, by the preponderance of leaves closely united 

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

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

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

 •han themselves, they are called Stolons, as, for instance, in J^gopodiiun Fodagraria, 

 Fragaria, Siruihiopteris gennanica, and in Mnium and Catharinea among Mosses. 



The greatest degree of variation from the ordinary forms of shoots is displa.yed 

 Dy the leaf-Uke flat shoots and systems of shoots, and by the stem-tendrils and 

 ;horn-like shoots which occur frequently in Angiosperms. Leaf-like shoots are 

 'ound in those Phanerogams in which large green foliage-leaves are wanting, and 

 replace them physiologically ; their axial structure is of considerable superficial 

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

 Lhey generally bear only very small membranous scale-leaves. Examples may be 

 found in Phyllocladus among Conifers, Ruscus among IMonocotyledons, and among 

 Dicotyledons in Miihleiihcckia platyclada (Polygonaceoe), Xylophylla (Euphorbiacese), 

 Carmichaelia (Papilionacese), Opuntia hrasilietisis, and Rhipsalis crispaia (Cacta- 

 ceoe), &c. 



The Stem-tendrils, like the 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 



