I'^H EXTERNAL CONFORMATION OF PLANTS. 



hyeniale, &c.), and in many cases the base of the leaf also, remains for a long time in the 

 condition of primary meristem, while the parts nearer to the apex, long since trans- 

 formed into permanent tissue, have attained their full growth. In this manner a 

 subsequent basal increase in length, often continuing for a long time, is occasioned in 

 parts which have long ceased to grow above ; this occurs in a peculiarly marked 

 manner in the long leaves of many Monocotyledons (Grasses, Liliaceae, &c.) which 

 are sheath-like in their lower part ; and to a smaller degree in many Dicotyledons 

 {e. g. Umbelliferae). Where, as in Ferns, and in a lower degree in many pinnate leaves 

 of Dicotyledons, the apical growth long remains active, the basal interstitial growth 

 usually soon ceases, and, vice 'versa, continues the longer the earlier the apical growth 

 comes to an end. Two extreme cases may therefore be distinguished in leaves, although 

 closely connected by intermediate forms ; the predominantly basifugal or apical and the 

 predominantly basal growth. 



If the interstitial growth continues at one part of the surface of the leaf, and attains 

 there a maximum which then decreases, a bag-like projection of the surface of the 

 leaf is formed, which is termed a spur, such as occurs in many petals (as Aquilegia, 

 Dicentra, &c.). 



(c) Before the tissues which are differentiated from the condition of primary mer- 

 istem assume their definite forms, a rapid growth usually takes place in their cells, which is 

 no longer accompanied by cell-division ; the size of the cells is not unfrequently increased 

 by this means ten or even a hundred-fold and more. This process, which is mainly de- 

 pendent on the rapid increase of the watery sap, may be termed Extension, in contra- 

 distinction to the growth of the younger cells which is contemporaneous with their divisions 

 and which always precedes the extension. On this extension depends the rapid unfold- 

 ing of the parts of the bud, which had long before been formed in their main outlines, 

 but had remained small. The buds very often remain a long time in a condition of rest, 

 until a rapid unfolding of the leaves and internodes already formed suddenly takes place ; 

 as, for instance, in the germination of many seeds, and in the persistent buds of many 

 trees (Aesculus), bulbs (Tulip), and corms (Crocus, &c.), formed in the summer and 

 germinating in the spring after long rest in winter. 



(d) The axis of length or growth of a member (as will further be shown in a special 

 paragraph), is an imaginary line passing from the centre of the base to its apex. The 

 entire growth both of leaves and of stems is usually most rapid in the direction of this 

 line; they are therefore for the most part longer than they are wide or thick. In stems the 

 growth is most often nearly equal along all diameters ; they assume therefore cylindrical, 

 prismatic, or bulbous roundish forms. It is, however, sometimes the case that the growth 

 in length advances much more slowly than that in diameter ; and then the stem becomes 

 tabular or fiat, as in many bulbs, the corms of Crocus, and especially in Isoetes. It is 

 only in the lateral shoots of higher plants with very limited growth that the internodes 

 grow mainly in the directions of a plane which includes the axis of length, and thus 

 become leaf-like, as in Ruscus, Xylophylla, &c. 



In leaves the growth usually preponderates in all the directions of a plane which cuts 

 the stem transversely, and is mostly symmetrical right and left of a plane which in- 

 cludes the axes of length both of the leaf and the stem ; the common form of leaves is 

 therefore that of thin plates symmetrically divided into two longitudinal halves. There 

 also occur, however, cylindrical and roundish tuber-like leaves, in \A7hich the growth has 

 been nearly equally rapid in all diameters at right angles to the axis of the leaf {e. g. 

 Mesembryanthemum echinatum). 



Sect. 22. Hairs (Trichomes) ^ is the term given in the higher plants to 

 those outgrowths which arise only from the epidermis, i. e. from the layer of cells 



^ Rauter, Zur Entwickelungsgeschichte einiger Trichomgebilde, p. 33. Vienna 1871.— Compare 

 also sects. 15 and 19 (b). 



