250 



II A R D W I C K E ' S SCIENCE-GOSSIP. 



LNov. 1, 1870. 



scendants during the year of its ordinary life ! 

 Correspondents who complain of the state of the 

 turnip crop should put their shoulders to the -wheel. 

 If children were employed to pick off the under 

 leaves of the turnips covered with aphides, to put 

 them in baskets and burn them without delay, much 

 good would result. Any fluid application for the 

 destruction of the insects seems hopeless, as they 

 invariably fix. themselves on the under side of the 

 leaves. — I. 0. W., in Gardeners' Chronicle and Agri- 

 cultural Gazette. 



ON THREE -LEAVED BRANCHES, CON- 

 SIDERED IN RELATION TO THE 

 THEORY OF THE ARRANGEMENT OF 

 LEAVES IN PLANTS. 



By F. V. Paxton, M.B. 



IN the October number of Science- Gossip, Mr. 

 Robert Holland alludes to a phenomenon which 

 has attracted my attention during the past summer, 

 on which I venture to offer the present communica- 

 tion, considering it to be not without interest, from 

 its bearing on the theory of Phyllotaxis. This pheno- 

 menon is the occurrence of whorls of three leaves 

 on opposite-leaved plants. I will take the liberty 

 of calling branches or plants on which it occurs 

 three-leaved, for the sake of shortness. 



"When examining the variations of the cotyledons 

 of seedling sycamores, the question occurred to me : 

 What would be the result, if a seedling with three 

 second leaves should attain to maturity ? Unfortu- 

 nately, it was then too late to try the experiment 

 during the present year. I was thus led to search 

 for whorls of three leaves in other opposite-leaved 

 plants. I was not long in finding that they are 

 common in many plants. Thus, I have found them 

 in the common nettle, hop, maple, Clematis vitalba, 

 horse-chestnut, ash, privet, wood-sage [Teucrium 

 Scorodonia), honey - suckle, cornel, and Aucuba 

 japotiica. 



This list might doubtless be easily extended. The 

 circumstances under which this variation occurs are 

 very uniform, and are such as would enable us to 

 predict the result which Mr. Aitken has actually 

 observed in a seedling sycamore. 



They are as follows : — 1st. Where a whorl of 

 three leaves is found in any axis of growth, the 

 whole of that axis is almost invariably found to be 

 three-leaved ; 2nd. The three-leaved arrangement 

 scarcely ever extends to secondary axes. The only 

 plant in which I have yet observed it so extending 

 is the Fuchsia, and in that not very commonly. 



I may here mention a species of purple balsam, 

 common in cottage gardens, which is invariably 

 three-leaved in its primary, and opposite-leaved in 

 all secondary ones. 3rd. Three-leaved shoots in 

 herbaceous plants always grow direct from the root; 



in shrubs and trees they are always vigorous spring 

 shoots, very frequently growing from stumps or 

 branches which have been cut back. 



In order to ascertain, if possible, the nature of this 

 variation, I have been led to examine more particu- 

 larly the shoots of Clematis vitalba, a plant in which 

 it occurs moderately often. This plant appeared 

 most suitable for the purpose, from the very definite 

 and uniform structure presented by its stems. On 

 examining a young shoot, it is found to be fluted. 

 In the more'usual opposite-leaved form, there are 

 seen to be six grooves dividing six projecting ribs. 

 Each of these ribs corresponds to a large vascular 

 bundle, and each groove to a much smaller one. 

 One rib on each side extends from the point where 

 the two expanded petioles meet, to the centre of a 

 petiole in the next joint. The other ribs are inter- 

 mediate in situation, and become continuous with, 

 and send spiral vessels to, the expanded margins of 

 the petioles next above them. Thus, the ribs of 

 any internode correspond in situation with the 

 grooves of the next. 



In the three-leaved shoots there are nine ribs and 

 nine grooves, their arrangement with reference to 

 the leaves being similar in every respect to the 

 ordinary form. The expanded edges of the petioles 

 form an uninterrupted line round the shoot. Hence 

 it would appear that, under certain circumstances, 

 probably mainly connected with excess of nutri- 

 ment, a bud is developed containing the elements 

 necessary for the production of whorls of three 

 leaves instead of two, this condition being, I pre- 

 sum e, that designated a trifoliar phy ton. 



Considered with reference to the theory of Phy4- 

 lotaxis, this variation would suggest some interest- 

 ing considerations. This theory appears to be 

 generally stated in treatises on botany, something 

 as follows:— "The formative power of leaves moves 

 in a spiral round the axis. This spiral is tolerably 

 uniform in any one species, and is capable of being 

 expressed mathematically. In order to extend this 

 theory to plants with opposite and whorled leaves, 

 it is usually assumed that in them one or more 

 internodes are suppressed at each joint. To cor- 

 roborate this view, the case is brought forward of 

 those young vigorous shoots of ash and some other 

 plants in which the leaves are not absolutely oppo- 

 site, but one leaf of the pair is somewhat in advance 

 of the other." 



This explanation has always appeared tome some- 

 what forced. It is difficult to understand why, in a 

 plant with whorls of seven or eight leaves, so many 

 internodes should be suppressed, and then one be 

 produced five or six inches in length. The exami- 

 nation of the three-leaved clematis shoot suggests 

 a further question : Why should the suppression of 

 two internodes instead of one, be regularly accom- 

 panied by an increase by one half of all the elements 

 of the stem ? 



