194 EXTERNAL CONFORMATION OF PLANTS. 



of acicular leaves) ; in Cycas scale-leaves alternate regularly on the stem with 

 large foliage-leaves. Seedlings (as in the oak) and the lateral shoots of under- 

 ground axes often begin with scales and only advance at a later period to the 

 production of foliage-leaves (e. g. Struthiopteris, ^Egopodium, Orchis, Polygonatum, 

 &c.). In parasites and plants growing on decaying vegetable matter (Saprophytes) 

 which are destitute of chloroph) 11, the scales are the only foliar structures of the vege- 

 tative parts, the foliage-leaves being absent («?. g. Monotropa, Neottia, Corallorrhiza, 

 Orobanche, (fee). Even in those plants whose foliage-leaves are much segmented 

 the scales remain simple ; they are distinguished by a broad base, usually diminutive 

 length, the absence of prominent veins, and by forming no chlorophyll or only very 

 little. They are colourless or yellowish, reddish, often brown ; their texture is, 

 according to circumstances, fleshy, succulent (as in some bulbs), membranous, or 

 tough like leather. 



In Phanerogams, especially in Monocotyledons and Dicotyledons, several other 

 forms of leaves make their appearance as a preliminary to fertilisation — Bracts, 

 Sepals, Petals, Stamens, and Carpels. The thick seed-leaves or Cotyledons will be 

 spoken of in detail as a peculiarity of these classes. 



From the point of view of the Theory of Descent we are justified in considering 

 all other forms of leaves as subsequent metamorphoses of foHage-leaves. These 

 latter are therefore regarded as the original typical leaves. When they lost their 

 original function — the assimilation of food-materials — and served other purposes, 

 they assumed at the same time other forms and other relationships of structure. The 

 same is meant when certain tendrils and thorns are termed metamorphosed leaves: — 

 Leaf-tendrils are leaves or parts of leaves w^hich have become filiform, and possess 

 the power of winding round slender bodies and thus of serving as climbing organs 

 (as in Vicia, Gloriosa, Smilax aspera, &c.). Leaf-thorns are leaves which have 

 developed into long, conical, pointed, hard, woody bodies ; they take the place of 

 foliage- leaves (Berberis) or represent metamorphosed stipules {Xanthhim spinosu?n, 

 some Acacias). These two kinds of metamorphosis occur almost exclusively in 

 Flowering Plants (Angiosperms), the morphological and physiological perfection 

 of which, in comparison to Cryptogams and Gymnosperms, is especially caused by 

 the capability of their leaves to assume the most various forms. 



(2) Forms of Shoots. The axis of leaf-bearing shoots is, when sufficiently 

 developed, usually columnar w^ith a cylindrical or prismatic surface. If the growth 

 in length is very small in proportion to that in thickness, the short column forms 

 a plate, the depth of which is shorter than its diameter, as in the bulbs of Allium 

 Cepa and Isoetes; if the growth in length is somewhat greater, with at the same 

 time considerable increase of thickness, rounded or elongated masses are produced 

 (as in the tuber of the potato and artichoke, the aerial stems of Mammillaria 

 and Euphorbia meloformis) ; when the growth in length greatly preponderates we 

 have stems, scapes, and filiform structures of various kinds. Very commonly the 

 same shoot shows differences of this kind in the successive segments of its longi- 

 tudinal growth ; thus the stem of the onion, which is at first broad and tabular, 

 afterwards rises as a high naked scape, the end of which in its turn remains short, 

 and thus produces the capitular inflorescence; and in the same manner the thick 

 tuber of the potato is only the swollen end of a slender filiform shoot. Among the 



