PHY LLOT AXIS. 



47 



which has its leaves distichous on the lateral boughs, so as to expose their 

 upper surfaces to the light from above, will produce vertical shoots from 

 the top of the bulb, having their leaves arranged on the f plan. The same 

 occurs with the free branches of the ivy compared with those climbing 

 up a wall, on which the leaves are distichous. 



The Yew seems to have its leaves distichous, but a closer inspection 

 reveals the fact they are not so, but on the f plan : each leaf, however, 

 is twisted at its base, so as to make the whole horizontal. The young 

 shoots, which may stand more nearly vertical, reveal the true order. As 

 this habit is retained in the Irish Y T ew the leaves are never distichous on 

 that variety. 



Floral PhyUotaxis. — As the parts of flowers are all identical in origin 

 with leaves it is natural to suppose that they will follow the same laws. 

 This they do, with certain modifications, in order to adapt them for 

 their new functions. We have seen why 3's prevail in monocotyledons, 

 and in dicotyledons o's. The reason is that 5's are cycles of the 

 § arrangement and 3's of the |. But, instead of each cycle, i.e. floral 

 whorl, being exactly over the preceding, the "law of alternation" now 

 intervenes ; so that each whorl alternates, i.e. in a "decussate " manner, 

 with the next. Hence when a whorl happens to be suppressed this 

 alternation appears to be violated, as stated in the case of the five stamens 

 of a Primrose. If 2's or 4's occur in flowers of dicotyledons, they 

 generally result from opposite leaves, as in Lilac. 



By the way the petals are folded in the bud various modifications 

 become revealed. If the reader draw a plan of the f arrangement on 

 a spiral containing two coils, and place the Nos. 1, 2, 3 at an angular 

 distance of 144°, he will find that by a slight alteration the second 

 petal (represented by a crescent) may have one edge underlying the third 

 petal. Then, if the fourth similarly underlies the fifth, we get the so- 

 called imbricate whorl, i.e. each petal overlaps the next. But when the 

 first is also underneath the third then the convolute or "contorted " condition 

 is reached, characteristic of mallows, flax, convolvulus, &c. 



Various modifications are to be seen in papilionaceous flowers of the 

 Pea family, and especially in flowers with coherent petals, as the Snap- 

 dragon, &c, but for details of these I must refer the reader to my paper 

 elsewhere.* 



Elaborate calculations as to the number of parts of flowers in many 

 different individuals of the same species have been made to ascertain 

 their relative sequence. Thus on counting the rays or pedicles of umbels 

 of Torilis Anthriscus the maxima were 5, 8, 10 (=2x5); while the 

 number of ray petals of Chrysanthemum segetum gave 13, 21, 26 (=2 x 13), 

 34. In the case- of the stigmatic rays of poppies 13 is the maximum. 

 With the ray flowers of the ox-eye Daisy, 21 far outstripped the others. 

 In the Cowslip, 3, 5, 8, 10(=2x5), 13 were the maxima, 5 being the 

 chief. 



It will be at once seen that these maxima correspond with the 

 different phyllotactical arrangements ; but why, in fixing the number of 

 parts in a whorl, Nature should endeavour to retain what may be called 

 " cycular " numbers is unknown. The above experiments in calculations 



* " On the Origin of Floral ^Estivations," Trans. Linn. Soc. vol. i. 1876, p. 177. 



