DEVELOPMENT OF LEAF-ORGANS. 599 



the varying direction and curvature of the organs of plants ; but this ex- 

 planation does not appear to be wholly satisfactory. The subject will be 

 further considered in subsequent paragraphs devoted to the effects of 

 gravity, geotropism, &c. 



Sect. 5. DEVELOPMENT AND GROWTH OF LEAF-ORGANS. 



Phyilomes (and all their metamorphosed forms, such as the parts 

 of the flowers &c.) originate laterally just beneath the absolute apex 

 of the stem, by cell-division of a single cell in Vascular Cryptogams, 

 of a group of cells in Phanerogams which results in the deflection 

 to one side of a small group of cells forming a conical papilla, or 

 the formation of an annular collar (sheathing-leaves), which deve- 

 lops into an independent lateral organ. These leaves arise inde- 

 finitely, or acropetally, the topmost being the youngest, in order 

 regulated by the laws of Phyllotaxy. The papillae from which 

 leaves originate are at first wholly cellular, consisting of periblein 

 covered by a layer of dermatogen cells ; after a time elongated cells 

 are formed in the centre ; and these are followed by spiral vessels 

 formed in a direction from the base upwards. As a rule, the first 

 part of the leaf formed is its point, which is gradually pushed out 

 by development at the point of junction of stem and leaf. The 

 apical growth of the leaf is generally soon arrested, but interstitial 

 multiplication also occurs in different parts of the leaf (especially 

 in stalked leaves). 



The pushing-out of the leaf by development at its base may be well 

 observed in the leaves of Hyacinth-bulbs developed in early spring. Not 

 only are the tissues (epidermis, &c.) younger below, but the relative 

 growth of the parts" may be demonstrated by making a series of marks at 

 equal distances up the" leaf and watching the proportionate extent to 

 which they become separated. The same process gives very instructive 

 results when applied to the measurement of the growth of the roots of the 

 same plants, and is easily carried out with bulbs grown in glasses of water. 



The basilar or basipetal mode of leaf -formation above described is that 

 which is most frequent ; but in some instances the apex of the leaf, instead 

 of early losing its power of growth, continues to grow and develop new 

 cells in that situation, the cells at the base of the leaf, in these cases, being 

 the oldest. This mode of leaf-formation is called basifugal. 



These modes of leaf -formation may be well seen in the case of lobed or 

 compound leaves. Thus in the Rose or Passion-flower the terminal leaflet 

 is first formed, and the lateral leaflets afterwards from above downwards, 

 according to the basipetal plan. In Mahonia and in many Leguminosa?, 

 such as the Garden Pea, the lower leaflets are formed first and the others 

 subsequently, according to the basifugal plan. The lobes or notches of 

 simple leaves are in like manner formed in one or the other of the methods 

 just alluded to. The stipules are often developed before the leaf -blade ; 

 when otherwise, they are probably lateral developments from the petiole 

 rather than separate organs. 



