OLD AND YOUNG LEAVES. 351 



still delicate blade hangs from it like a closed parasol, as in Podophyllum, Cortusa, 

 Hydrophyllum, and several Ranunculacese. In the Horse-chestnut {JEsculus 

 Hippocastanum) the folded leaflets are erect when they emerge from the bud; they 

 then sink down so that their apices point to the ground ; and later, when the 

 epidermis has become more thickened, they again rise until they are almost 

 I horizontal. Leaves of limes (Tilia grandifolia and parvifolia) are vertical when 

 they first break through the bud, the apex directed towards the ground; it is only 

 later that they become almost parallel with its surface. The upright leaf-stalk is 

 often bent like a hook at the end, and the vertical folded leaflets depend from the 

 hooked portion. This arrangement is shown in the common Wood-sorrel, and 

 j many other plants (see fig. 90 ^). 



A third method of protecting these delicate undeveloped green portions of 



I young leaves consists in the formation of screens and coverings, which exhibit the 



. greatest variety. The envelope is frequently furnished by the so-called stipules. 



In many plants two lobes arise on the right and left of the leaf-stalk at the point of 



j junction of the leaf and stem, and these have been termed " stipules " (stipulce). In 



: figs, oaks, beeches, limes, magnolias, and numerous other plants, the stipules are 



I membraneous, pale, usually without chlorophyll, and they appear like scales placed 



as screens in front of the small, tender green leaflets when they burst through the 



bud, and in any case must be considered to protect them from the sun's rays (see 



! fig. 92). When once the young leaf has grown beyond the top of these screens and 



I no longer needs them, they shrivel up, are detached, and fall to the ground. 



j Millions of such fallen scales, called in botanical terminology " deciduous stipules " , 



! are to be seen on the ground in oak and beech forests shortly after the leaves have 



j attained their normal size. The stipules of magnolias, particularly of the Tulip- 



! tree (Liriodendron tulipifera), a native of North America, but now cultivated all 



j over Europe, are very remarkable (see fig. 91). They are comparatively large and 



boat-shaped, and are always so arranged in pairs as to form a closed cup. Shut 



up within this membraneous, slightly transparent cup can be seen the young leaf, 



its stalk being bent into a hook, and the two halves of the blade folded togethet 



along the midrib like those of the Cherry, In this position the leaf grows gradually 



as if in a small greenhouse ; it enlarges, and as soon as the epidermal cells are so 



much thickened that there is no further danger of it drying up, the cup opens and 



the two boat-shaped stipules separate from one another, shrivel up, and at length 



fall off. Only two scars at the base of the leaf remind one that two stipules were 



situated here in the spring, whose function was to protect the delicate young leaf 



from desiccation. 



One of the most noticeable arrangements for the protection of the tender, 

 undeveloped green tissue consists in the peculiar grouping of the leaf -veins. This 

 may be best observed in foliage-leaves which are folded along the lateral veins in 

 vernation. Each individual leaf is erect, usually a little bent at the apex and 

 margins, and slightly hollowed so that the upper surface is concave, and the lower 

 side, which is turned towards the incident light, convex. Since the midrib of the 



