252 



HUTCHINSON'S POPULAR BOTANY 



The arrangement of the mature and developed leaves on the stem is 

 also worthy of attention. To regard the mass of foliage on a tree as an 

 orderly arranged series of organs might seem to be a far-fetched thought : 

 yet order reigns in nature where the unpractised eye sees only disorder. 

 It was long ago remarked by Charles Bonnet, an eminent Swiss naturalist 

 of the eighteenth century, that leaves and their modifications have normally 

 a spiral arrangement on the stem. The fact (for the truth of the obser- 

 vation is beyond question) is more easily understood of the foliar than of 

 the floral leaves, and may be better seen in some plants than in others. 



It is spoken of as phyllotaxy. 



The leaves of a Cherry-tree (Cerasus) will 

 furnish a suitable illustration. Here (fig. 313) 

 is a piece of a branch with all the leaves be- 

 longing to it. "We will number them in their 

 order of growth, 1, 2, 3, 4, 5, and 6. Xow for 

 our spiral. Commencing at number 1. draw a 

 chalk line from the base of the leaf to the 

 base of number 2, and from thence to the 

 same point in leaf 3, and so on, to the base 

 of each leaf in succession till number 6 is 

 reached. See now what has happened ! The 

 chalk line has traversed in a spiral manner 

 exactly twice round the branch, and the be- 

 ginning of the line at number 1 is exactly 

 under the end of the line at number 6 ; or, in 

 other words, the first leaf corresponds verti- 

 cally with the sixth. Had the fragment of 

 branch been longer, and contained eleven 

 leaves instead of six, we should have found 

 on continuing the line in the same manner 

 that is, from base to base of the additional 

 leaves that the point of the chalk would 

 have travelled, as before, twice round the 

 branch in order to reach number 11. More- 

 over, and as a consequence, the leaf specified 



would have been found to be in the same vertical line as 1 and 6. As to 

 the other leaves, number 7 would have been found to be over number 2, 

 8 over 3, 9 over -1. and 10 over 5 in fact, the interesting discovery would 

 have been reached that the leaves are disposed on the branches in cycles 

 of five : and the way would have been cleared for the statement that the 

 laws which regulate the foliar arrangement of all plants, and the floral no 

 less than the foliar, may be reduced to the same mathematical precision 

 (fig. 313). 



Not, of course, that the leaves of all plants fall under the same arrange- 



FIG. 309. Bryophyllum 



calycinum. 

 Formation of buds at t'ae edges of a leaf. 



