ON THE MORPHOLOGY OF THE CALYCULUS B y C. Ve n k a t a R ao Department of Botany, Andhra University, Waltair (Received for publication on July 30, 1962) I ntroduction T h e subfamily Loranthoideae of Loranthaceae is characterized by the presence of an annular, toothed or lobed structure below the perianth called the calyculus. The morphological nature of the calyculus has been much discussed and so far no satisfactory explanation has been offered. Roxburgh (1874), Bentham and Hooker (1862-93) and Daiiser (1931) considered it to be equivalent to the calyx. However, some doubt was being felt regarding its equivalence to the calyx because the structure is nonvascular in several genera, e.g., Dendrophthoe (Narayana, 1954), Macrosolen (Maheshwari and Bahadur Singh, 1952), Helkanlhes (Johri, Agrawal and Garg, 1957) etc. Narayana (1955) working on Nuylsia florihunda found that the 3-7 lobed calyculus in this species is vascularised by 5 bundles which depart at the base of the floral stele. Maheshwari, Johri and Dixit (1957) who devoted much attention to morphological and embryological studies in the family conclude: “ while further work is necessary, it now appears likely that the calyculus may represent the calyx". Other taxonomists, however, disagree with the above view. Eichler (1878) considered it to be an outgrowth of the floral axis. Engler and Prantl (1897) and Engler and Krause (1935) accept this view. Schaeppi and Steindl (1942) drew attention to the fact that the lobing of the calyculus when present does not bear any relation to that of the perianth whorl. Willis (1948) remarks that to consider the calyculus as an outgrowth of the axis is ‘perhaps the safest view’. However, it is unlikely that a structure which is fed by vascular bundles aiising from floral stele like other floral parts, should be an organ sui generis without any morphology. The possibility of the calyculus being a reduced or vestigial whorl of bracts of a suppressed lateral branch inflorescence was never sug­ gested because the flowers in Loranthoideae are stalked and individually provided with bracts which are ‘always united to their stalks up to the point of origin of the flower’ (Willis, 1948). O bservations Loranthaceae is a family which has undergone great specialisation and reduction in vegetative and floral organs in consequence of the para­ sitic habit. The ovules have completely disappeared and in some mem­ bers a structureless mass of meristematic cells arises from the floor of the loculus called the mammelon. The presence of isolated tracheids in this structure in some genera like Dendrophthoe, Amyema and Nuytsia led Maheshwari et al. (1957) to consider it as a reduced placenta] tissue. These authors couid construct a series of stages leading to the formation of mammelon and even to its complete disappearance. The presence of vascular bundles in the calyculus in some genera and their absence in others also indicates that the calyculus is a vestigial struc­ ture. The morphological nature of such an organ cannot be decided by ontogenetic or floral anatomical studies. Only studies in com­ parative morphology and evolutionary trends in the inflorescence and flower of allied families can throw some light on the morphology of the calyculus. Several taxonomists suggested affinities among Loranthaceae, Proteaceae and Santalaceae. Bentham and Hooker (1862-93) place the families Loranthaceae and Santalaceae in the order Achlamydo- sporae of the Monochlamydeae and the family Proteaceae in the adjacent order Daphnales. Likewise Engler and Prantl (1897) and Rendle (1952) place the families in the adjacent orders Santalales and Proteales. Recent moiphological, floral anatomical and embryological studies in the three families have brought to light a number of close resemblances among them, particularly between l.oranthaceae and Proteaceae. The flowers are uniformly tetramerous in Proteaceae as they are in Viscoideae of Loranthaceae and in Santalaceae. The flowers occur in threes in Santalaceae and Loranthaceae or they become paired due to the suppression of the central flower. Paired arrangement of the flowers is characteristic of all genera of the subfamily Grevilloideae of the Proteaceae. The flowers in Loranthaceae and Proteaceae are pro­ vided with an elongated perianth which becomes brightly coloured and conspicuous. The tepals are valvate in aestivation. They do not show tapering margins as the perianth parts do in other families but are uniformly thick. When polyphyllous they unite in bud due to the interlocking of marginal hairs (Text-Figs. 2, In Loranthoideae the perianth splits down on one side (Hutchinson, 1959), which feature is quite common in Proteaceae (Text-Figs. 4, 7). The perianth segments in Proteaceae and Loranthoideae become strongly reflexed as the flower opens. In Loranthaceae, Proteaceae and Santalaceae the stamens are equal in number to the tepals and antetepalous. The filaments are adnate to the tepals. The anthers are biloc'ular and introrse and show longi­ tudinal dehiscence. In Loranthoideae and Proteaceae the connectives of the stamens are commonly produced beyond the anthers (Text- Fig. 1). B-io The anther tapetum in all the three families is of the glandular tyt>e and c/tokinesis of the microsporocytes is brought about by simultaneous furrowing. The pollen grains are triangular, oblately flattened, tripo- rate with the germ pores situated at the angles (Erdtman, 1952). They are two-celled at the shedding stage. The embryo-sac in all the three families develops according to the Polygonum type. In Loranlhoideae commonly several megaspore mother cells function. In Synaphea of the Proteaceae also a large number of functional megaspore mother cells is seen and sometimes more than one megasporc of a tetrad function (Venkata Rao, 1961). Endosperm hausloria are found in Proteaceae and Santalaceae (Manasi Ram, 1957; 1959 a, 1959 Bhatnagar, 1960). In all the three families there arc nectaries which are equal i.n num­ ber to the tepals and alternate to them, or sometimes united into a ring. These have been considered by Haber (1959) and Eames (1961) to be equivalent lo reduced petals in Proteaceae. Eames (1961) remarks: "the nectariferous disc of some genera of Proteaceae (four separate glands i i other genera) is from the evidence of comparative form and anatomy vestigial corolla and this family can no longer be placed in the supposedly primitive Apetalae” . But the present writer has produced arguments why the glands cannot be regarded as morpho­ logically equivalent to petals (Venkata Rao, 1960). In some genera of Proteaceae, Brabeium, Macadamia and Lamhertia the nectaries are in the form of a small tube or cup surrounding the base of the ovary. This necessitates the presence of an annular meristem inner lo the lepal- sfamen whorl. To say that a ‘gamopetalous corolla' arises inner to an epiphyllous staminal whorl is tantamount to disregarding all morpho­ logical tenets. The nectaries in all the three families, therefore, seem lo be merely nectar secreting outgrowths of the thalamus similar to those of the dichlamydeous flowers and not equivalent to the petals. The flowers in Loranthoideae and Proteaceae show close resem­ blance in anatomical structure. The traces for the perianth segments are organised as 4 or 5 strands from the floral stele. Close to their origin the staminal traces arise as pairs of bundles to the inside of the tepal traces. These run parallel lo each other and to the tepal bundle to varying heights (i.e., the tepal and stamen show congenital concres­ cence). The two staminal bundles fuse together before their emer­ gence into the filament. Such a feature is described by Johri et al. (1957) in Helicanifies and by the present writer in a number of Proteaceae (Venkata Rao, 1960, 1961). (Compare Text-Figs. 2 and 3 with Text- Figs. 16 and 18 of Johri ei at., 1957.) The organization of the staminal trace as a pair of bundles is quite rare in angiosperms and the similarity of this feature in the two families taken in conjunction with other features of resemblance should point to a close affinity between them and to the essential homology of the perianth. However, quite conflict­ ing views have been expressed regarding the morphology of the perianth in the two families. Hutchinson (1959) who regarded the calyculus as a reduced calyx refers to the coloured perianth of the Loranthoideae as the coroJla while in the Proteaceae he described the perianth as the calyx. Eames (1961) who considered the glands in the Proteaccae as reduced petals regarded the coloured perianth in this family as sepals. The close similarity in morphological, histological and anatomical features of the perianth in the two families shows that it is essentially homologous and there is little justification for discriminating it as corolla in one family and calyx in the other. Thus the families Loranthaceae and Proteaceae (ajid Santalaceac to some extent) show resemblances in morphological, histological, anatomical and embryological features which indicate that they are probably derived from common monochlamydeous ancestors. The present studies show that evolution of the inflorescence in the Loran­ thaceae and Proteaceae has progressed along parallel lines. A study of the evolutionary trends in Proteaceae which has not undergone reduc­ tion to such an extent as in Loranthaceae is bound to throw sorte light on the morphology of the calyculus. V, The present writer (Venkata Rao, 1957) has shown that the primi­ tive inflorescence in Proteaceae is a much branched, lax panicle. Two trends of evolution are noticed from such a type; (o) reduction in the number of flowers of the lateral branches to two or one and (b) con­ densation of the inflorescence by reduction of pedicels and/or peduncles either before or after reduction in the number of flowers in the lateral branches. These tendencies arc also noticed in several other angio- spermous families (Eames, 1961) like Palmae (Bosch, 1947; Venkata Rao, 1959), Gramineae, etc.