PHYLLOTAXIS. 



45 



be developed between the leaves of each pair. It subsequently becomes 

 equal to the longer intervals between the pairs. Thirdly, each pair grows 

 at an obtuse angle of about 150°, finally becoming between later pairs of 

 leaves 144°, when the f arrangement is established. 



Returning to fig. 3, it will be observed that if a spiral line be drawn 

 through the numbers 1 to 6 it will make 2 coils or circles : 1 to 9 will 

 make 3, 1 to 14 5 coils, and so on. 



It will be noted that \ and \ are not provided for : yet these two 

 represent the commonest arrangements in Monocotyledons, just as the 

 preceding do in Dicotyledons. The reason for this is that in the latter 

 class alternate leaves follow on a pair of opposite cotyledons ; and the 

 usual plan is f , as may be well seen in a germinating acorn. 



When there is only one cotyledon the next leaf may be either at a 

 distance of 180°, that supplies the \ ; or 120°, which results in the 

 \ arrangement : because (with the exception of -X) in no case is there ever 



2 



5 

 10 

 13 

 18 

 21 



3 8 11 16 19 20 15 12 7 4 



22 = / r 

 17=2 xj 

 14=t% 



6-§ 

 1 > 



Fig. 3. 



more or less than three leaves in any part of the spiral which completes a 

 circle.* 



Species of Pondweed (Potamogeton) have their leaves distichous, but 

 not infrequently they are in pairs, apparently opposite toithout being 

 decussate ; but one leaf of each pair really overlaps the other. They may 

 be distichous with internodes well developed between each leaf (P. htcens) ; 

 or there may be some in pairs, others alone (P. prcelongits, perfoliatiLS, 

 and crispus) ; or they may all be in pairs (P. densus). 



It is an interesting fact that Banunculus Ficaria, which has one 

 cotyledon only, also has its leaves distichous in a similar manner. But 

 when we arrive at the flowers we find the phyllotaxis suddenly changed 

 to J arrangement in the 3-sepalled calyx, and | in the 1-petalled corolla. 

 This will be explained below. 



That Monocotyledons are descended from Dicotyledons is borne out 

 by the fact that it is not infrequent to find the embryo of the former pro- 

 vided with a rudimentary second cotyledon. Thus, Tamus communis, the 



* In the series \, &c. there are always 4 leaves &c. 



