Polytrichum psilocorys 153 A NOTE ON THE PERIODICITY OF LEAF- FORM IN TARAXACUM OFFICINALE

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1 Polytrichum psilocorys 153 sterilized by boiling and kept in a glass box. They germinated abundantly and the culture remained pure. The young moss-plants appeared on the protonema, but they showed an extraordinarily slow growth and I never succeeded in getting any sporogonia from them, though they were kept alive for more than five years. HORTUS BOTANICUS, AMSTERDAM, August, A NOTE ON THE PERIODICITY OF LEAF- FORM IN TARAXACUM OFFICINALE BY B. MILLARD GRIFFITHS, D.Sc, F.L.S. (With 14 figures in the text) IN May, 1920, fruits were collected from a specimen of Taraxacum officinale found growing in the grounds of University College, Reading. The plant had large spathulate leaves which were about 50 cm. long and 11 cm. broad (Fig. I). It is not possible to state the age of the plant but subsequent evidence will show that it might be as much as three years old. Fruits of the plant were sown in a pot in the greenhouse of the Botanical Department, Armstrong College, Newcastle-on-Tyne, on April 13, By May 6 two seedlings appeared and these were planted out in the open in July. All the leaves produced were of the markedly dentate type (Fig. II), and continued to be so throughout the winter. In the spring, slightly larger, but still markedly dentate leaves began to appear (Fig. III). In May, 1922, the plants flowered (indicated by F in figures), and the leaves produced subsequently became less and less dentate, until by July, large, broad and spathulate leaves were developed (Figs. IV- VII). From August onwards the reverse process took place, until by the winter the leaves were once more of the markedly dentate kind. The season of 1923 opened with the production of markedly dentate leaves as before, but they were of larger size (Fig. VIII). This type was maintained until the flowering period in April, immediately after which the form of the leaves began to change once more. From June to August, leaves of an increasingly less dentate form appeared, until finally they were broadly spathulate and over 40 cm. in length (Figs. X and XI). With the coming of autumn, the leaves began to decrease in size and increase in dentation, until by October they were

2 154 B. MiLLARD GRIEEITHS only some 12 cm. long and nearly as dentate as in the previous winter stage (Figs. XII and XIII). II III IV F V VI VII VIII IX 7^ X XI XII XIII XIV Fig. I. Leaf of parent plant. Kigs. II-XIII. Successive leaves of filial plant over a period f>f two years, II-VII 1922, VIII-XIII l'ig. XIV. Extremely dentate leaf from another plant. F indicates flowering period. The figures are from pliotographs of pressed specimens. Unfortunately at this point building operations made it necessary to transplant the speeimens, and serious damage was unavoidably done to the roots.

3 Periodidty of Leaf-form in Taraxacum officinale 155 Growth experiments were also conducted on T. erythrospermum. Fruits were collected from typical specimens found growing in sandy ground near Reading, and also from the sand-dunes at Seaton Sluice, Northumberland. These were germinated and grown at Armstrong College, but it was found that as far as the experiments were carried, the offspring came true to type, and there was no change in leafform as in the case of T. officinale. The periodic changes in leaf-form raise two questions, one of which is physiological and the other taxonomic. Priestly and Pearsall have shown that in the case of certain plants, it is possible to change the leaf-form by changing the pressure of the water-supply. When the pressure was increased, a young leaf which was normally lobulated, tended to develop into a less lobulated and more entire form, while diminution of water-pressure led to the lobulation of leaves which were normally entire (Brit. Assoc. Report, 90th Meeting, Hull, 1922, P- 394)- If the pressure of the water-supply is causative in the matter of leaf-form, T. officinale appears to be a species which shows large variations in its water-pressure throughout the year. On this interpretation, the pressure of water in the plant is low in the winter, and the leaves are consequently dentate, but after the flowering period, there is a progressive increase in pressure until the autumn, and the successive leaves are less and less dentate, until in the summer they become spathulate. On the same hypothesis, there is a rapid fall of pressure in autumn, and the leaves revert relatively rapidly to the low-pressure dentate form. It may be pointed out that the leaves of Taraxacum are very thin and watery, wilting takes place within a few minutes of cutting, and there is very httle sclerenchymatous tissue. The rigidity of the leaf is maintained mainly by the turgor of the cells, and it is therefore not improbable that the leaf is peculiarly sensitive to changes in water-pressure. The taxonomic aspect of these observations is also of interest because the form of the leaf is of importance in the division of the species into varieties. In Babington's Manual of British Botany, ioth ed. 1922, the varieties are: T. officinale Vill. Rchb., with leaves runcinate and broad; T. laevigatum D.C, runcinate pinnatifid with unequal teeth; T. erythrosperm,um D.C, runcinate pinnatifid with unequal teeth and intermediate smaller ones; and T. palustre D.C, oblong and entire sinuate-dentate or runcinate leaves. One might suggest that possibly in T. erythrospermum we have a segregate of T. officinale, a segregate which has a water-pressure

4 156 B, MiLLARD GRIFFITHS variation-curve on one side of the original type mean, and that T. palustre is an analogous segregate with a variation curve on the other side of the mean. An extreme variation of T. officinale itself is seen in the case of the leaf shown in Fig. XIV, taken from another plant found in the grounds of University College, Reading. The dentation is extreme and resembles that of T. erythrospermutn but on a much larger scale. Two attempts were made to germinate the fruits of this plant, but without success. No valid conclusion can be drawn from this failure, but it may be pointed out that if such an extreme variant is normally sterile, T. erythrospermutn^ may have arisen as a very rare fertile variant of the type, and that this variant had a low-pressure habit of water-supply, and was consequently able to colonize the sandy spots in which it now flourishes. As Taraxacum is an apomictic genus, the segregation of a variant is all the more likely. DEPARTMENT OF BOTANY, ARMSTRONG COLLEGE, NEWCASTLE-ON-TYNE. January, D; THE CELL WALL IN THE RADICLE OF VICIA FABA AND THE SHAPE OF THE MERISTEMATIC CELLS BY R. M. TUPPER-CAREY AND J. H. PRIESTLEY (With I figure in the text). URING investigations into the composition of the cell wall at the 'apical meristem of the root(i), it was noticed that when the cells of a dry radicle of the broad bean were macerated under suitable conditions, the walls showed a very definite and constant structure. Sections of radicles previously soaked in water, then treated for 24 hours with concentrated ammonia, followed by 24 hours in» T. erythrospermutn also differs from T. officinale in the characteristic colour of the fruit. No suggestion is here made as to the nature of the correlation between this character and the leaf-form.

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