LEAVES. 



55 



branches of the primary are called secondary, and the further 

 subdivisions of the veins arc called veinlits. 



Evciv leaf is by no means constructed wilii all ilie above 

 parts. The petiole is ot'ten \vaniii;f:, when the leaf is said to 

 Iw sessile and the midrib is ot'ten undistinguishable from the 

 veins, but the above gives the general type of leaves and the 

 variations wdl be noticed in their proper plact s. 



61. The arranjien ent of the leaves on the stem is various 

 but in the same SMCcits it is unilorni; a beautifid symmetry 

 is established m every variety. Sometimes they are arrang- 

 ed in opposite pairs, wiih one pair at right angles with the 

 pair above or below it : at others they alternate, wiili one 

 alMjve the other, on nearly opposite sides of i he stem. The 

 alternate leaves, however, are generally arranged in a spiral 

 torui. 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 arranjred alternately, he will notice 

 the second leaf above or below the one observed, does not 

 come immediately above or below it ; but he must pass several 

 pairs betbre he will find one corresponding exactly with the 

 one first noticed. On the cherry, or Allhea, for instance, he 

 will pass two pairs before he will tind one exactly over the 

 one observed, here two turns of the spire take place before the 

 generating point corresponds with the one below it. Oppo- 

 site leaves sometimes brcome alternate, but we believe, that 

 they are always exactly on the opposite parts of the stem, 

 never terming the spiral arrangement of common alternate 

 leaves; and the cause of this alternation is undoubtedly the 

 unequal development of the two sides of the stem. It some- 

 times haj)p(.'ns that several opposite jiairs are developed on 

 the same horizontal section of the stem and are called ver- 

 ticihte, but the basis of the leaves of any whorl are not im- 

 mediately under the basis of the leaves of the next wliorl 

 above it, but those of the second whorl correspond with them, 

 so that of four whorls the basis of the first and third corres- 

 pond, and the second and fourth and so on. The Pine pre- 

 sents a striking example of the spiral arrangement of organs. 

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

 we shall readily discover, that they are arranged spirally ; 

 and by cutting off the leaves composing one spire, we shall 

 find, that they do not form a single spire, but a compound one 

 consisting of three or four spires runninjr paralh 1 to each 

 other. The above are the different arrangpnjcnts of leaves, 

 but there seems a tendency in oppc site and verticilate leaves 

 to assume the spiral arrangement, without, however, vars ing 



