LEAVES AND LEAF-BEARING AXES. 1 59 



In by far the greater number of leaves the apical growth ceases early, the apex 

 itself becoming transformed into permanent tissue. In Ferns, however, the apical growth 

 of the leaves usually continues, and in many genera is even unlimited, the apex of the 

 leaf always remaining capable of development, and not becoming transformed into 

 permanent tissue, as in Nephrolepis ; in Gleichenia, Mertensia, Lygodium, and Guarea, the 

 growth of the apex of the leaf is, as in many shoots, periodically interrupted, and again 

 renewed in each period of growth. 



(b) Besides the apical growth, there always exists, however, both in stems and in 

 leaves, an interstitial growth, by which the parts produced by the apical growth increase 

 in size and become further developed. The development of the internodes of the stem 

 depends almost exclusively on this interstitial growth, as indeed is shown by the crowded 

 position of the leaves and the shortness of the internodes in the bud ; it is generally at 

 first very rapid, and the increase in size occasioned by it is often very considerable ; but 

 it usually soon ceases, and the tissues become differentiated into unchanging permanent 

 forms. Not unfrequently, however, a basal zone of the internodes (as in Grasses, Equi- 

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

 in a condition capable of development, while the parts nearer to the apex, long since 

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

 a secondary 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. Umbelliferas). 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 apical and the predomi- 

 nantly 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 

 meristem 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 dependent on the rapid increase of the watery sap, may be termed Extension, in 

 contradistinction to the growth of the younger cells which is connected with their 

 divisions, and which always precedes the extension. On this extension depends the 

 rapid unfolding of the parts of the bud, which had long before assumed their main out- 

 lines, 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 (horse-chestnut), bulbs (tulip), and corms (crocus, &c.), formed in 

 the summer and germinating in the spring after long rest in winter. 



(d) The Axis of gro^vth or of length of a member (as will further be shown in a special 

 paragraph) is an imaginary line passing from the centre of the base to the 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 broad or thick. In 

 stems the growth is most often nearly equal along all diameters ; they assume therefore 

 cylindrical, prismatic, or bulbous rounded 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 flat, as in many bulbs, the corm of the crocus, and especially in 

 Iso'ites. It is only in the lateral shoots of higher plants which have a very limited 



