348 OLD AND YOUNG LEAVES. 



forced its way out of the bud above the soil, or from between the cotyledons, the 

 conditions are still the same, and therefore particularly efficacious protective 

 arrangements are required that the leaves just merging from the bud, and thus 

 exposed to the vicissitudes of the weather, may grow up properly, i.e. that their 

 green transpiring tissue may be normally developed. 



Some of these protective contrivances belong exclusively to the developing 

 period of the leaves, and are lost when they become fully grown. Others may be 

 seen in the adult leaves. The most striking instances are perhaps the diminution 

 of the surfaces directly exposed to the sun and wind, the vertical inclination of 

 the leaf-blades, and the concealment of the green tissue under a protective mantle. 



The diminution of the surface directly exposed to the sun and wind is caused by 

 the position which the foliage-leaf takes up within the bud. Space is very limited 

 here, and the youngest and smallest leaves appear to be fitted into the space by 

 the rolling, or folding, or crumpling of their blades. This diminution is obviously of 

 great advantage when the leaves open out into the daylight: it constitutes a special 

 protection against the drying up of the green tissues, and is, therefore, retained 

 until other protective measures are developed, and in some cases even throughout 

 life. In many Polygonaceae (e.g. Polygonum viviparum and Bistorta), in species of 

 Butter-bur (Petasites), in some Primulacese, and especially in many bulbous plants, 

 the green portions of the leaf are rolled. The midrib, and frequently a fairly broad 

 central strip of the leaf in addition, remains flat, and the right and left halves are 

 rolled up from the margins, sometimes towards the upper, sometimes towards the 

 lower surface. The stomata are chiefly, or wholly, to be found on the concave side, 

 beneath which lies the soft green tissue with its ramifying air-passages. In the 

 Crocus, the two halves of the leaf are rolled outwards; they are connected togethei 

 by a broad, white, central stripe which is not rolled, and is devoid of chlorophyll; 

 in the Star of Bethlehem (Ornithogalum), whose leaves are traversed by a similar 

 white stripe, the leaf margins are rolled inwards. In species of Crocus the stomata 

 are placed in the two grooves on the under surface; in the Star of Bethlehem, in 

 the grooves on the upper surface. The central stripe of the young leaves in the 

 plants mentioned always remains flat, but in young fern-leaves, which are also 

 rolled, the strongly-developed midrib is curled spirally inwards like a watch-spring, 

 and thus the green feather-like pinnae, springing from the rachis, are placed one 

 above the other. Most ferns in their native habitat rarely require special protec- 

 tion against over- transpiration during the first stages of development; but -when 

 this is necessary, it is afforded in every case by the form assumed by the young 

 leaf just described. Moreover, in such instances special protective envelopes are, 

 as a rule, to be found, which will be spoken of later. 



Leaves are not so often crumpled as rolled on first emerging from the bud. In 

 crumpled leaves the net- work of anastomosing veins forms a strong lattice-work, and 

 the green leaf-substance, fitted into the interstices of the lattice, is swollen up like 

 bubbles or sunken into pits, giving the whole leaf the appearance of a crumpled sheet 

 of paper or cloth. The vernation (or position occupied by the leaf in the bud) is 



