350 OLD AND YOUNG LEAVES. 



veins becomes the midrib of a leaflet, as in the cinquefoils, and species of clover 

 and Wood-sorrel. Each leaflet is folded up along the midrib like a sheet of paper, 

 and the folded leaflets are placed side by side in the same way as folded leaves 

 in a book. 



When the leaves are pinnate, and the leaflets are arranged in pairs on a common 

 rachis, the latter are folded together along their midribs, and placed side by side, so 

 as to resemble the pages of a book. This vernation occurs in roses, Mountain Ash 

 (Sorbus aucuparia) and Walnut (Juglans regia), see figs. 90^ and 90*. In the 

 roses the rachis is so short in the bud that the leaflets springing from it appear to 

 originate from one point, as in the cinquefoils. In most maple-leaves and those of 

 Saxifraga peltata, the folding takes place not along the radiating veins alone, but 

 along the short lateral veins which spring from the larger radiating ribs. In this 

 way small folds are inserted between the larger, and this vernation leads up to that 

 which was described before as " crumpled ". The leaf -folding exhibited by the 

 foliage of the Beech (Fagus silvatica, see fig. 92), the Hornbeam and the Hop- 

 hornbeam (Garpinus and Ostrya), the Oak (Quercus), and many other plants, whilst 

 in the bud, is very characteristic. Each foliage-leaf possesses a midrib and numer- 

 ous strong lateral veins, which run right and left from the midrib like the bony 

 processes from the spinal column of a fish. The green portions of the leaf form 

 deep folds between these lateral veins, which are as yet very close to one another, 

 and the folds are thus arranged exactly as in a fan. Yet another method of folding 

 occurs in the Cherry (Prunus avium). Each leaf, while in the bud, and for some 

 time after it has burst from it, is folded along the midrib only (see figs. 90 ^ and 90 ^). 

 The right and left halves are so flatly folded together, and fit over one another so 

 completely, that at first sight they appear to form only a simple leaf -blade. More- 

 over, the two halves which are in contact are actually joined by means of a balsam- 

 like secretion. At this stage of development they are always erect; and this 

 brings us to another protective contrivance to be observed in young undeveloped 

 leaves. 



It may be stated that, with the exception of a few " crumpled " forms, all young 

 foliage-leaves when they emerge from the bud-scales, or from between the coty- 

 ledons, or as they force their way through the soil into the light of day, are so 

 directed that their blades are not horizontal. In this first stage of development, 

 indeed, the green transpiring, but still delicate, portions of the leaf have always a 

 vertical position. Their blades usually exhibit the direction observed in phyllo- 

 clades and phyllodes, in the equitant leaves of irises and tofieldias, in the leaves of 

 the compass plants during their greatest activity, and in the leaves of grasses 

 when folded together in dry air. Sometimes the entire extended or rolled blade is 

 erect, as in most bulbous plants and grasses; or the midrib is inclined towards the 

 horizon, in which case the halves of the leaf are folded together and the two 

 margins come into contact, forming a sharp edge which is turned towards the sun 

 at noon. This is seen in some grasses (Glyceria, Poa), and in the Cherry (Prunus 

 avium). If the blade is not erect, the stalk of the leaf is perpendicular while the 



