14 STRUCTURAL BOTANY 



rather close attention to make out. We see at once 

 that no two leaves arise at the same level. Every 

 leaf is placed a little above or below its next neigh- 

 bours. The leaves, therefore, are said to be alternate. 

 We find further that they are arranged in a spiral. 

 An imaginary line connecting the bases of the suc- 

 cessive leaves together, would wind upwards round 

 the stem, just like the thread of a screw ; here, 

 however, the spiral ascends from the right up to the 

 left, like a left-handed screw, in the reverse direction 

 to an ordinary right-handed one. 1 Now, if we count 

 the leaves carefully, beginning from below, and not 

 counting the leaf taken as our starting-point, we shall 

 find that the fifth leaf comes exactly above the one 

 we started with, and in order to reach this fifth leaf 

 we have had to follow the spiral exactly twice round 

 the stem. The ridges make it much easier to see 

 which leaves come in the same straight line ; for, as 

 we saw just now, each ridge runs straight from one 

 leaf to that vertically above or below it. This 

 arrangement of the leaves may be expressed by the 

 fraction f, in which the denominator indicates the 

 number of the leaf which comes directly above the 

 starting-point, while the numerator tells us how 

 many times the spiral line travels round the stem 

 before the fifth leaf is reached. This fraction 2 also 

 expresses the divergence or angle between two success- 

 ive leaves, which is equal to | of the circumference 

 of the stem. The way in which leaves are arranged 

 on the stem is called phyllotaxis, and varies very 

 *nuch in different kinds of plants. 



1 In the diagram (Fig. 6) the spiral ascends from left to right 



