8TBUGTUBE OF LEAVES 33 



centre in all directions, and usually go beyond the circumference of the 

 foliage. In this way the active fibrous roots secure the full benefit of 

 the rain which is thrown off the leaves. 



A complete leaf consists of a flattened portion, called the blade ; a 

 stalk ot: petiole,, and a pair of more or less scaly or leafy appendages 

 called stipules, all of which characters are illustrated in the Glossary 

 at fig. 98. The stalk and stipules are frequently absent, leaving the 

 blade attached to the stem by the base, when it is said to be sessile. 

 As a rule, when fully developed, leaves cease to increase in length or 

 breadth, and remain unchanged in form until death, when they are 

 removed by natural decay, as inmost Monocotyledonous plants (p. 127), 

 or by breaking off at a joint, as in most Dicotyledons (p. 122). 



Some leaves develop and die in one season, and are called deciduous ; 

 others persist for two or more seasons before falling off, new ones in 

 the meantime being formed, thus giving the plant an evergreen appear- 

 ance. 



The tissue of the blade is traversed by a framework of stronger ribs 

 or veins more or less netted in Dicotyledons, as shown at fig. 48 in the 

 Glossary, p. 7, and parallel or curved in Monocotyledons, as shown at 

 fig. 96 in the Glossary. The beautiful arrangement of the veins is 

 well seen in leaves which have been skeletonised — that is, when the 

 more perishable tissue (called parenchyma) between the veins has 

 decayed through artificial or natural agencies. These veins serve 

 not only to strengthen the leaf-blade, but also as channels through 

 which the sap from the root is distributed to the cells composing the 

 blade. 



Structure of Leaves.— Ordinary leaves consist of an upper and 

 imder surface between which are layers of cells more or less irregular 

 in shape and filled with grains of green colouring matter known as 

 chlorophyll floating about in the protoplasm (see p. 22). The cells 

 near the upper surface are much more compactly arranged than those 

 beneath, hence the more intense green of leaves above. Where leaves 

 receive an equal amount of light, as in Irises, Gladiolus, and many 

 other Monocotyledons, there is not a great difference in colour between 

 the two sides. 



The skin, cuticle, or epidermis of the leaf is studded with small 

 openings or breathing pores, knovTn as stomata. These are more 

 numerous on the under, surface, and it has been computed that there are 

 as many as 60,000 of them to the square inch in a Lily leaf, and about 

 100,000 to the square inch on that of an Apple leaf. The leaves of water 

 plants present a striking difference from those of land plants. Those 

 under water are more or less divided and are so thin in texture that they 



