50 



THE FLOWERING PLANT. 



dicotyledons, the two first leaves (the cotyledons or seed-leaves) 

 are opposite, however the following leaves maybe disposed (fig. 2). 

 Seedlings of mustard and cress show this very well (fig. 3). 



When the leaves are alternate, they are arranged so that their 

 angular distance from one another is constant. That is to say, 

 if the stem were telescoped so as to bring any two successive 

 leaves to the same level, and if two radii were drawn from the 

 centre of the stem at that level through the insertions of the two 

 leaves, the angle enclosed by the radii would be constant, generally 

 speaking, for the same plant. Diagrams of phyllotaxis represent 

 the stem as being extremely conical, and looked at from above, 

 the leaf insertions being marked by thick curves. The simplest 

 case is that of grasses, &c, where any particular leaf is succeeded 

 by one placed on the opposite side of the stem. The divergence is 

 evidently 180 , and this may be expressed by the fraction J, i.e., 

 half of the entire circumference of the stem, or 360°. If a line 

 is drawn round and round the stem in the same direction, cutting 

 the insertions of the leaves, it will form what is called the genetic 

 spiral. Of course a spiral line might be drawn according to 

 pleasure in either direction, right to left, or the opposite. It is 

 agreed, however, always to take the shorter course. Starting 

 from any leaf and proceeding upwards, the spiral will wind once 

 or more round the stem, till a leaf is reached immediately above 

 the first, i.e., in the same rank or orthostichy. Such a portion 



of the spiral is called a cycle. 

 The numerator of the fraction 

 J shows us that the cycle takes 

 one turn round the stem, while 

 the denominator signifies that 

 the cycle contains two leaves, 

 and that there are two ortho- 

 stichies. The series J, J, f , 

 |, T 5 y, &c, includes the com- 

 monest divergences. From § 

 onwards the numerator and 

 denominator of any fraction 

 are respectively the sums of 

 the numerators and denomi- 

 nators of the two preceding 

 fractions. The two-ranked (J) 

 arrangement not only occurs 

 in grasses, but also in many 

 other monocotyledons, and in 

 The three-ranked Q) distribu- 

 tion is characteristic of sedges, and the alder and aspen. Most 



FIG. 9.— I Phyllotaxis of Cherry. 



twigs of the lime, elm, beech, &c. 



