44 LEAVES. 



the bundles of vascular tissue enter the lamina, they divide, and 

 proceed in various directions in different plants, but always in 

 the sairre manner in the same species, forming the veins (Fig. 

 38, c) of the leaves. The continuation of the petiole forms the 

 middle and largest vein of the leaf, called the midrib (Fig. 38, d). 

 Those veins arising from the midrib are called primary, the 

 branches of the primary are called secondary, and the further 

 subdivisions of the veins are called veinlets. 



The petiole is often wanting when the leaf is said to be sessile, 

 and the midrib is often undistinguishable from the veins. 



79. The arrangement of the leaves on the stem is various, 

 but in the same species it is uniform : a beautiful symmetry is 

 established in every variety. Sometimes they are arranged in 

 opposite pairs, with one pair at right angles with the pair 

 above or below it ; at others they alternate with one above the 

 other, on nearly opposite sides of the stem. The alternate 

 leaves, however, are generally arranged in a spiral form. They 

 are not on exactly opposite sides of the stem. The student 

 will observe, that by taking any leaf on a branch on which the 

 leaves are arranged alternately, he will notice that the second leaf 

 above or below the one observed does not come immediately 

 above or below it ; but he must pass several pairs before he 

 will find one corresponding exactly with the one first noticed. 

 On the Cherry, or Althea, for instance, he will pass two pairs 

 before he will find one exactly over the one observed. Here 

 two turns of the spire take place before the generating point 

 corresponds with the one below it. Opposite leaves sometimes 

 become alternate, but we believe that they are always exactly 

 on the opposite parts of the stem, never forming the spiral ar- 

 rangement of common alternate leaves ; and the cause of this 

 alternation is undoubtedly the unequal development of the two 

 sides of the stem. It sometimes happens that several opposite 

 pairs are developed on the same horizontal section of the stem, 

 and are called verticillate, but the bases of the leaves of any whorl 

 are not immediately under the bases of the leaves of the next 

 whorl above it ; but those of the second whorl correspond with 

 them ; so that of four whorls the bases of the first and third 

 correspond, and the second and fourth, and so on. The Pine 

 presents a striking example of the spiral arrangement of organs. 

 If we examine the extremity of a branch covered with leaves, 



What is the midrib ? What arc primary veins ? Secondary? When are 

 leaves said to be sessile? — 79. What is said of the arrangement of leaves? 

 How are alternate leaves arranged ? Illustrate it. How in the cherry f 

 How do opposite leaves become alternate 'i How are whorls formed f 

 How in the pine ? 



