ABNORMAL specimens showing concrescence of the two needle

Transcription

1 L. A. Boodle. 19 CONCRESCENT AND SOLITARY FOLIAGE LEAVES IN PINUS. BY L. A. BOODLE, F.L.S., Jodyell Laboyatory, Royal Gaydens, Keis. [WITH FOUU FIOURES IN THE TEXT.] ABNORMAL specimens showing concrescence of the two needle leaves on a spur shoot were discovered by Strasburger' in two species of Pinus (P. Puinilio and P. sylvestyis), and were described by him in connection with his interpretation of tbe morpbology of the double needles of Sciadopitys veytkiuata. A similar abnormality has heen found on a tree of Austrian Pine' growing in the Royal Botanic Gardens, Kew. Most of the foliage of this tree is normal, but pairs of coucrescent leaves are produced every year in fair numbers. The nature of the fusion between the two leaves is illustrated by the transverse section shown diagrammatically in Fig. 1. This section was cut near the base of the double leaf, viz. at a height of 4 mm. above the basal sbeatb of scales. The structure is identical with that of two normal leaves of the same species, except for the differences immediately connected with the fusion, the mesopbyll of tbe two leaves being continuous at m, and tbe epidermis and bypoderm [ep. hy.) respectively being continuous on eitber side of ;;;. Tbe fusion described above may be continued from the base to near the apex of the two leaves, but never quite to tbe apex, two free leaf-apices being always present. At a short distance below the point at which these tips become free, the tissue connecting the two leaves shows a decrease in breadth, which continues until, in the last transverse section showing concrescence, the leaves are only united by one or two epidermal cells at tbe extreme matgin. Measurements of twenty-five specimens of double leaves gave 9'5 mm. as the average length of the free portion, and 89-0 mm. for the concrescent portion. In one case the free part was only 1 mm. long, and in another ease it was 47 mm. long, these being tbe two extremes observed. The examination of a considerable number of twigs on the tree on which the abnormality was found, gave tbe result tbat one or more double needles are present among the tuft of needles at the end of nearly every twig. The leaves on the spur-shoots on six 1 Strasburger, " Die Coniferen und Gnetaceen," 1872, p linus Laricio Poir. var. nigricaiis Parl. (=/=. Larieio var. austl'iaca Endl.)

2 Jo L. A. tioodle. twigs were counted, and the numhers of leaves were as follows. There weie 602 normal pairs of leaves,' 20 double leaves and 6 solitary leaves, i.e., 6 leaves each borne singly on a spur-shoot.= One of the features of this case of concrescence is its simplicity. It is clear that only the two normal leaves are represented, and there appears to be no necessity to assume that any of the tissue in the region of concrescence is other than leaf-tissue. At any rate in a case like the present one, in which the apical growth of the shoot becomes arrested, it may be held that redaction in the bulk of the apical cone might involve the suppression of some Fig. 1. Fig. 3. A. B. C. L. FIGS Austrian Pine {Pinus Laiicio Poir., var. nigvicans Parl.). FIG. 1. Transverse section of double needle : ep. Iiy., epidermis and bypoderm ; m., mesopbyll; r.c, resin-canal; e., endodermis; xy., xylem ; ph., phloem. X about 30. FIG. 2. Diagrams illustrating variable orientation of the double needles: (f-, twig or parent axis ; L, scale-leaf borne by the twig ; ;(., double needle borne by the spur-shoot in the axil of I. A, adaxial fusion; B, abaxial fusion ; C, oblique orientation. 1-iG. 3. Transverse section of a solitary leaf, showing the groove, g. X about 30. FIG. 4. Transverse section through the base of the same leaf, showing two protuberances, apparently the apex of tbe spur-shoot, a., and the rudiment of the second leaf, I. X about 30. ' Normal except that in a few cases one leaf was a good deal shorter than the other. These solitary leaves will be described later.

3 Concrescent and Solitary Foliage Leaves in Pinus. 2 i tissue which normally separates tbe bases of the two leaf rudiments. The latter would then be continuous, and a concresent structure would result, which may be regarded as entirely foliar in origin, unless visible indications to the contrary can be adduced. The fusion always extends less than half way across the transverse section of the double leaf, and, when the hases of some specimens were examined, a small papilla, apparently representing the apex of the short shoot, was found hetween the partially fused leaves. The orientation of the double leaves is not constant. Cases were found in which the leaves were fused by their adaxial margins, i.e. the margins directed towards the twig, and away from the scaleleaf in the axil of which the spur-sboot with its two foliage-leaves is borne (Fig. 2, A). Other cases show fusion of the abaxial margins (Fig. 2, B), and obliquely placed-concrescent leaves also occur (Fig.2,C). It appears quite probable that the douhle needles of Sciadopitys verticillata may be morphologically similar to tbose of the Austrian Pine descrihed above ; that is they may represent two foliage-leaves fused by their margins. On this assumption tbe two leaves in Sciadopitys would be united by tbeir adaxial margins, as shown by tbe orientation of the vascular bundles. Hence tbe double leaves of Seiadopitys would agree, as regards orientation, only with such double leaves of tbe Austrian Pine as correspond to diagram A in Fig. 2. Neither a spur-shoot nor a separate apex representing it can be recognised in Sciadopitys, so the double needle of this plant may be regarded as the result of fusion of two foliage-leaves belonging to a spur-sboot, which has been suppressed. Tbis view was held by H. von Mobl.' Other theories must be recognised as tenable, e,g. that of Strasburger,' who regarded the douhle needle as the result of the concrescence of two leaves witb one another and witb tbe apex of the spur-shoot. GoebeP interprets the douhle needle as a leaf-like twig or phylloclade, bearing on its primordium the tips of tbe two needles as small points. A teratological specimen, which was regarded hy Bower* as giving support to this view, should be referred to bere. The specimen owed its origin to the proliferation of a cone of Sciadopitys, and resembled a double needle at its base, but was 1 H. von Mohl, Bot. Zeit., Vol. 29 (1871), p Strasburger, loe. eit., p = Goebel, " Organography of plants," Eng. ed.. Part 2 (1905), p " Bower, Gard. Chron., N. S., Vol. 21 (1884), p. 346 ; see, for figure, p. 282 of the same volume.

4 2 2 Coficrescent and Solitary Foliage Leaves in Pinus. forked into two brancbes above. One of these bore higher up an apparently simple needle and also a scale-leaf with a small twig in its axil. Of tbe six solitary leaves of the Austrian Pine mentioned ahove three were of quite normal form, and a withered stump of the second leaf was found on each of the three spur-sboots. Tbe three other solitary leaves were abnormal, being rotighly cylindrical in form.' In two of these a groove was present on one side of the leaf, and was continued upwards for some distance from the base. It is seen at g in Fig. 3. On following the groove downwards in a series of transverse sections, it was found to contain, right at tbe base of the leaf, two papillae, one of which (a in Fig. 4) appears to represent the apex of the spur-shoot, and the other (/ in Fig. 4) to be the arrested rudiment of the second leaf. In tbe third specimen the leaf showed no groove except at the extreme base, and in this case only one papilla was found. In Pinus inonophylla Torr. the spur-shoots mostly bear each a single needle, hut two are occasionally present. When the leaf is solitary, it is roughly cylindrical, hut when two leaves are present, each is nearly semicircular in transverse section. Masters^ found by studying early stages tbat two leaf-rudiments are always produced, hut that one of them generally becomes arrested at an early stage. Hence the solitary leaves of Pinus monophylla are strictly comparable with those found on the Austrian Pine. Arrest of one leaf on a spur-shoot, which is a rare occurrence in the latter species, has become general in tbe former. It is not unlikely that hoth double and also solitary needles may occasionally occur on otber species of Pinus" besides those mentioned here. As the abnormal needles are by no means conspicuous among the normal foliage, they are liable to escape notice. A double needle of Pinus, like tbe examples described ahove, may be regarded as being produced by partial fusion of two normal leaves, though other views are not refuted. The normal needle of Sciadopitys may be somewbat similarly interpreted, but as showing more complete fusion of the two leaves, with suppression of the spur-shoot and its scale-leaves. Solitary needles, occasionally found in some species of Pinus, originate in the same way as the typical leaves of Pinns monophylla. ' Solitary leaves similar to these appear to have been seen by Strasburger {loc. eit., p. 389) in Phtiis Pumillo. ' Masters, " Pinns moiwphyllci." Ann. Bot., Vol. 2 (1888), p A few cylindrieal solitary leaves ha/e oeen found on a tree of Piims jnnriccifa D. Don.

5