160 JOUENAL OF THE EOYAL HOETIC ULTUEAL SOCIETY. 



They are, of course, the more important, but external agencies un- 

 doubtedly play some part in bringing about these changes. Masters has 

 given a full discussion of this question in the appendix on double flowers, 

 but the peloric foxglove may also be cited as an example. The more so, 

 as the influence of external causes is most easily studied in this case. 

 Our best way is to compare the terminal flowers on the side branches of 

 a plant, with the pelory at the summit of its main spike. On the weaker 

 branches, the peloric flowers become more and more regular. They lose 

 their association with other alterations and tend to become pentamerous. 

 This, of course, would be the condition of a normal flower of the fox- 

 glove, were it not for its irregular structure. Or, in other words, we may 

 imagine the ancestors of the species as having had pentamerous and 

 regular, instead of irregular, flowers. This philosophical conception is 

 now seen to be supported by the abnormal flowers on the weakest branches. 

 Here the corolla has lost only its symmetry. It is regularly cupshaped 

 with five small, but equal, lobes. Inside there are five stamens of the 

 same length and a normal ovary. 



In comparing side branches of different vigour, we can easily collect 

 a whole series of transitional forms between the two extremes just 

 described. 



The peloric foxglove is an hereditary variety. Its seeds have been 

 on the market for more than half a century. It was made an object of 

 botanical inquiry, for the first time, by the Dutch botanist, Vrolik, whose 

 preparations may still be seen in the botanical museum of the University 

 of Amsterdam. 



Spiral torsion is the subject of another of the chapters of Masters' 

 book. One of its most interesting instances is afforded by the twisted 

 stems of the wild teasel (figs. 45 and 46, d). Such stems are met with in 

 the field, from time to time, and from their seed I succeeded in starting an 

 hereditary race. It is highly variable and may show the deviation, under 

 appropriate cultivation, in all its individuals, but only part of them will 

 succeed in twisting their main stem over the whole length. 



In the normal stem of the teasel, the leaves are arranged in pairs, 

 which alternate with one another and are separated by long and straight 

 internodes, covered with longitudinal ribs. In the twisted stems, on the 

 other hand, the internodes seem to be absent, and the leaves are arranged 

 along a line, constituting a continuous band. This line spirally ascends 

 the stem ; its curves are low and numerous in the basal part, but soon 

 become steeper, and not rarely even quite erect. In this case, all the 

 leaves of the main part of the stem are seen to be directed to one side only, 

 thereby giving an extraordinarily strange appearance. 



The spiral torsion of the stem corresponds with this arrangement of 

 the leaves. The ribs, which are longitudinal in normal specimens, are 

 themselves twisted. The direction of their spiral is opposite to that of 

 the leaves. 



A similar twisting of the stem is, from time to time, met with in 

 other plants, the leaves of which are arranged in pairs or in whorls. 

 The horse-tail (fig. 46, c) and the cleavers (Galium Aparine) (fig. 46, b) 

 afford the most common instances. Ordinarily, only a small part of their 

 stem is spirally contorted, the lower and the upper parts being of normal 



