CHARACTERISTIC FORMS OF LEAVES AND SHOOTS. igc 



numerous deviations from the columnar form of the axis the conical is of peculiar 

 interest. The conical stem is of two kinds ; it may be slender at the base, increasing 

 in thickness with further growth in length, so that each portion of the axis is thicl 

 in proportion to its youth ; and the upright stem resembles a cone placed upon iti 

 point. The growing apex lies on the surface which is turned upwards, or risei 

 above it as an upright cone. This form occurs in the stems of Tree-ferns, Palms 

 and very clearly in the maize and in many Aroidese ; it depends on the absenc( 

 of a subsequent growth in thickness, while, as its age increases, the young tissu( 

 of the stem becomes constantly larger in circumference immediately beneath it 

 apex ; when this increase of strength at last ceases the circumference of the late 

 increment of length remains the same, and the inverted conical stem continues t( 

 grow above in the form of a cylinder. The second form of conical stem is cause( 

 by a long-continuing subsequent growth in thickness together with the small circum 

 ference of the shoot at the puncium vegelatwnis ; this occurs in Conifers and mam 

 dicotyledonous trees, the older stems of which are thick below but slender above 

 and thus resemble a slender cone placed on its base. 



The habit of a shoot or of a segment of a shoot is usually in close relatioi 

 to the number, size, and formation of its leaves. If the internodes are very short 

 but the leaves small and numerous, the surface of that portion of the axis is nowhen 

 exposed, and the leaves only are seen, as in species of Thuja and Cupressus, anc 

 some Mosses (Thuidium); in such cases whole systems of shoots frequently hav( 

 the appearance of multipinnate leaves. If the closely packed leaves are large, the] 

 form a rosette enveloping the end of the stem, while the older parts of the sten 

 are clothed with the remains of the leaves, or are naked, as in Tree-ferns, manj 

 procumbent stems of species of Aspidium, many Palms, species of Aloe, &c. 



If a comparison is made between the amount of development in bulk whicl 

 takes place in the leaves and in the axis of a shoot, we find as extremes on om 

 side, for example, the Cacti (Cereus, IMammillaria, Echinocactus, &c.) with gigantic 

 axes and entirely abortive leaves, on the other side the Crassulacese with flesh} 

 crowded leaves and comparatively weak stems ; or on one side the undergrounc 

 tubers of the potato with scarcely visible scales, and on the other side the bulbj 

 of Liliaceae with fleshy scales which entirely envelope the short stem. 



In reference to the formation of leaves which appear on the shoots, it musi 

 first be noted whether the same axis always produces only simiTar leaves or such 

 as gradually vary in form. The first is the case, for example, in most Mosses, 

 Ferns, Lycopodiaceae, Rhizocarps, all Equisetacese, and most Conifers ; the latter. 

 on the other hand, occurs commonly in shrubby Dicotyledons. In Monocotyledons 

 and Dicotyledons (to a certain extent even in Conifers) it not unfrequently happens 

 that the difl"erent forms of leaves are distributed over different generations of shoots ; 

 certain shoots produce, for example, little or nothing but foliage-leaves, others 

 produce only bracts with or without flowers {e. g. Begonia). In such cases the 

 shoots may be designated, according to their leaves, scaly shoots, leafy shoots, 

 bract- axes, flowers, peduncles, &c. On this point further details will be given in 

 Book II. 



It is of very common occurrence with Cryptogams and Angiosperms (not 

 with Gymnosperms) for a persistent primary axis or system of shoots to continue 



o 2 



