CHAPTER VI. 



FOLIAGE AND SCALE LEAVES. 



MORPHOLOGY. 



The ordinary green leaves of a plant are known as Foliage Leaves. 

 They vary very greatly in shape, but in all of them certain parts 

 can be recognized, which are known by distinctive names. Exa- 

 mine, for instance, a shoot of a garden geranium. Each leaf 

 will be seen to present two distinct parts, the leaf stalk or petiole, 

 and the expanded blade or lamina supported by this. The leaf is 

 horizontally directed, with upper surface (back) facing the stem, 

 and lower surface (front) turned away from it. A vertical plane 

 passing through the stem will bisect the leaf into two corre- 

 sponding halves. This plane is called the median or antero- 

 posterior plane, and the leaf is said to be bilaterally symmetrical. 

 In other words, it can only be divided by one plane so as to give 

 corresponding halves, which in this case may be termed right and 

 left. Each of them is, so to speak, the reflection of the other, 

 i.e., if one half of the leaf is placed with its cut edge against a 

 mirror, the reflection will resemble the missing half. Optically 

 speaking, each half resembles the other half laterally inverted. 

 The typical stem, on the other hand, is radially symmetrical, in 

 that a number of planes can divide it into corresponding halves. 

 There is consequently no distinction between surfaces and sides, 

 right and left. 1 In a three-sided stem there are three such 

 planes, in a four- sided one four, and in a smooth cylindrical stem 

 an infinite number. The lamina, being the important part of 

 the leaf, is rarely absent (see p. 53), but the petiole is frequently 

 so, and the leaf is then said to be sessile, as in the majority of 

 monocotyledons, and in many dicotyledons. This is probably a 

 more primitive condition, and all gradations are found between 

 it and cases where a stalk is present. That is to say, the boundary 

 between lamina and petiole is not always sharp. The latter may 

 be winged, there being a thin green strip on either side of it, 

 directly continuous with the lamina. And if this strip is fairly 

 broad, we no longer speak of a winged petiole, but of a narrow 

 region of the lamina. There are, however, still other regions in 

 certain leaves. A blade of grass will be found at its base to be 

 1 Horizontally-directed stems are bilaterally symmetrical. 



