r984l N. G. AND R. E. DENGLER:PLICATION FORMATION Principes,23(1),1984, PP. 31 48 Formation of Plications in the Pinnate Leaves of r11 r'I t Lnrysalldocarpus Iutescens and the PalmateLeaves of Rhapisexcelsa NaNcv G. DrxcrsR ANDRoNALD E. DEncrnn Department oJ'Botany, finiuersity ofToronto, Toronto, Ontario MSS lAl, Canada and Department of Botany, Scarborough College, Uniuersity oJ'Toronto, Vest HilI, Ontario MIC lA4, Canada For over 100 years plant morphologists palmately compound leaves first appear as have known that the large, dissectedleaves hood-shaped protuberances which encircle of palms exhibit a developmental pathway the shoot apical meristem. A series of which is distinctly different from that of transverse to oblique folds or plications the compound leaves of all other flowering develop near the margins of the primor- plants. ln those dicotyledonsand mono- dium (Fig. lA); these indicate the position cotyledons having dissected leaf blades, of the future leaf blade. A distinctive {ea- leaflets arise as free lobes on the margin ture of these plications is that they do not of the primordial leaf. In contrast, leaflet extend to the leaf margin, leaving an inception in the palms occurs by folding unplicate marginal strip of tissue. During of the submarginal part of the lamina, fol- the growth of most palm leaves a localized lowed in most groups by the secondary separation of tissue results in the splitting splitting of the blade into individual leaf- oT-the lamina into individual leaflets. In lets. Since this mode of leaflet origin seems some groups of palms the splitting occurs to have no counterpart in other flowering in the ridges nearest the shoot apex (Fig. plant species and is markedly complex in lB,C). This is the adaxial side of the leaf character, it has received a considerable and because the leaf bends away from the amount of attention from morphologists in shoot tip as it expands from the crown, the last century (von Mohl 1845, Hof- the mature leaflets are trough-shaped in meister 1868, Goebel 1884, 1926, Nau- cross section or induplicate. In other mann 1887, Deinega l89B) and more groups of palms, the splitting occurs in recently (Yampolsky 1922, E{mes 1953, the ridges farthest from the shoot apex Venkatanaray ana 1957 , Periasamy 1962, (abaxial ridges); this results in leaflets that Padmanabhan1963, 1967, Corner 1966, are ridge-shapedin crossseclion or redu- Padmanabhan and VeerasamY 1973). plicate (Fig. lD). In some palms, the loca- Despite the length of time that the prob- tion of the zones of separation does not lem has been studied and the large num- correspond to adjacent ridges so that indi- ber of papers devoted to this subject, con- vidual leaflets consist of a number of the troversies on the mechanism of the original original plications (Fig. lE). The tips of folding or plication remain unresolved and the leaflets also separate from the unpli- misinterpretations Persist. cate marginal strip of tissue which is In the past there has been general ephemeral in many groups, but which may agreement amongst these investigators on form a pair of conspicuous reins attached the general pattern of palm leaf morpho- to the two basal leaflets(Eames 1953). genesis. Both pinnately compound and The source of controversy in the liter- PRINCIPES lVoL. 28 X A Aboxiol Adoxiol B Adoxiol ---x ^--- Aboxiol UU (A) and the aPpearanceot a I. Diagramsillustrating an adaxialview of a hood-shapedpalm leaf primordium by the line (x x)' Abscissionthrough section(B) made through the ""u*"tgi""r plications in the plane indicated (C); through the abaxial ridges (arrow) will *" "J".irf ridges (arrJw) will result in iniuplicate leaflets abscission regions may result in leaflets consisting ,".Ji"."arpi"utl l"ufl"t" (D); or zonesoiabscission in thg intercostal of more than one Plication(E). such a way that ature has been over the mechanism of nal separation of cells in break through inception of the original folds or plications internal schizogenous slits on alternate sides in a previously smooth lamina. The point the epidermal surface appearance of disagreement is whether tissue separa- of the leaf giving the pleated the differences tion is involved in the initiatiqn of the pli- (Fig. 2A,D,E). At first might appear cations themselves. For example, Hof- between these alternatives quite dif- meister (1868), Goebel (1884, 1926), trivial; however, each involves If tissue Deinega (lB9B) and more recently Per- ferent morphogenetic processes. internal cells of the iasamy (1962) and Corner (1966) con- separation did occur, schizogenous clude that the plications are formed by a ground tissue lining the to become process of differential growth followed by spaces and normally destined cells would now r foldi.te of the meristematic lamina as piotosynthetic mesophyll of the leaf and shown in Figure 2A-C. On the other hand, L" o.r ih" external surface as the protective cells von Mohl (1845), Naumann (I887), would differentiate contrast, the process Yampolsky (1922), Eames (1953) and of the epidermis. In only involves local- Padmanabhan(1963. I967) argue that of differential growth to give rise to the the process of pleat formation involves not ized zones of growth of tissue with- only differential growth, but also the inter- adaxial and abaxial ridges l 9841 N. G. AND R. E. DENGLER:PLICATION FORMATION 33 A Adoxiol A boxiol ) I C EV 2. Diagrams illustrating the formation of alternating adaxial and abaxial ridges through diferential growth (A,B,C) or through the extension of internal schizcgenous slits to the surface (A,D,E). out the complicated redifferentiation of lutescensH. A. Wendl. (arecoid major cells. group, Moore I973) having pinnateredu- The purpose of this study is to resolve plicate leaves and Rhapis excelsa this long standing controversy over the (Thunb.) Henry (coryphoidmajor group, mechanism of plication inception and, Moore 1973) having palmateinduplicate particularly, to test the hypothe;is of the leaves.Shoots of both specieswere col- schizogenous origin of the plications and lectedat FairchildTropical Garden,Coral subsequent redifferentiation of tissue. Since Gables,Florida and were shippedto the this mechanism has not been reported to University of California, Berkeley. This occur in any other flowering plant groups, investigation was carried out in conjunc- this would be a further unique feature of tion with Dr. D. R. Kaplan, University of palm leaf development that might be of California, Berkeley, and the observations potential significance in evaluating the which are summarizedbelow were reported systematic relationships of palms, partic- in greater detail in three papers in the ularly with other monocotyledons having Canad.ianJournal of Botany (Kaplan et plicate leaves. al. I982a,6, Dengleret al. 1982). The Species having the two basic types of specific methods used to study the prob- palm leaf morphology were selected for lem of plicationinception are alsodescribed this investigation: Chrysalidocarpus in thesepapers. 28 34 PRINCIPES lVoL. Results Morphogenesis iz Chrysalidocarpus lu- tescens. The mature leaf of Chrysali- docarpus measures about 2 meters in length and is differentiated into a pin- nately compound blade, a short petiole and a smooth, tubular base (Fig. 3). Each of the approximately 80 leaflets has a single major vein or rib and is reduplicate. The leaves are arrapged in either a clockwise or counterclockwise phyllotaxis (Kaplan el al. I9B2a). The developing leaves are tightly packed together within the apical bud and their appearance in scanning electron micrographs and in sectioned material viewed in the light microscope suggest that considerable mutual pressure is exerted between adjacent leaves (e.9., Fig. 6). Figure 4 shows a young leaf about 0.5 mm in length in which there is no external indication of plication inception but which clearly shows the hood-shaped lamina, and a sheathing leaf base which encircles the shoot apex and younger leaf primordia. The upper limit of the sheath of the next oldest leaf can be determined by the loca- tion of the constriction near the base of the leaf lamina (Fig. 4, arrow). Developing 3 leaves of Chrysalidocarpus are charac- One margin of teristically asymmetrical. 3. Mature leaf oI Chrysalidocarpus lutescens the lamina protrudes through the sheath showing the pinnate lamina, petiole and tubular of the next oldest leaf; this is the anodic sheathing base. Scale line equals 50 cm. (*) margin. The other margin is appressed against the rachis; this is the tathodic (-) margin. The leaf illustrated in Figure 4 is from a shoot with counterclockwise phyl- ridee formation on the abaxial side of the lotaxis and therefore the right hand mar- anodic ma.gin; the folding of the cathodic ein as viewed from the adaxial side is margin against the rachis obscures the anodic (*). Leaves from shoots with ridges (Fig. 5A,B). As the leaf develops, clockwise phyllotaxis have the anodic (*) the abaxial surface appears flat in surface marein on the left. view and the furrows between the abaxial Plications first appear in leaves 0.6 to ridees resemble slits incised in the surface 1.0 mm in length as a series of slight of ihe lamina (Fig. 6). Plications do not ridges and furrows having a vertical ori- extend to the margin or apex of the leaf, entation in the distal part of the lamina leaving an unplicate margin and a hood- (Fig. 5A). Formation of the ridges on the shaped leaf apex (Fig. 6). While the first adaxial side of the leaf is seen to precede formed apical plications have a vertical 35 N. G. AND R. E. DENGLER:PLICATION FORMATION leaves of chrysalitlocarpus Lute'scens'Fig' 4' Leaf pri- 4,6. Scanning electron mrcrographs of developing (B), hood shaped lamina-(L) with anodic (*) and mordium 0.5 mm in length. NJte'th" sheathing l""uf ba." 'l'he formed by the sheath of the.next oldest leaf' Scale cathodic (-) margins.