Some fundamental considerations on the ‘New morphology’

Abstract

Summary Some questions discussed in and conclusions arrived at in the above paper are: 1. 1.Paleobotany changed the face of Morphology. 2. 2.Did Life develop out of the inanimate world in one single period or has it done so continually? 3. 3.Did the taxonomic ‘levels’ (categories of increasing size and discontinuity) now recognised rise at one given period, or do they continue to widen their scope? 4. 4.Why are ‘intermediates’ in paleontology so rare? 5. 5.Two elementary processes in the evolution of plants are: (more or less slow) growth alterations and (more or less sudden) differentiation. 6. 6.Differentiation primarily arises from unequal cell-division; growth changes are functions of tissues (meristems), due to physiological causes. Both are genetically determined. 7. 7.The concept of homology rests on inequality of daughter-cells of a cell-division and, though more vaguely, on the comparison of organs to which growth alterations are ascribed (cf. point 28). 8. 8.The concept ‘leaf’ has no morphological basis; at best it is a functional notion (cf. point 20). 9. 9.The basic unit in the Cormophyta is the protostelic telome. In all but the most primitive types, all nerves are homologous with mesomes, nerve endings with telomes or mesomes. Most organs can only be interpreted as homologa of other organs when at least some of them are vascularised. 10. 10.Dichotomy is the only and universal way of ramification in Cormophyta except perhaps in the very oldest ones. The possibility of an original lateral ramification is discussed. Types of ramification, traditionally described or named differently are derivable from the original dichotomy by the assumption of some simple evolutionary processes to which can be ascribed a selectional value. 11. 11.Mixed Protective Bifurcating Units (MPB Units) — a dichotomy with a fertile and a sterile part — arose early in the evolution of stachyosporous Cormophyta and acquired a considerable autonomy. They were either mono-or poly-telomic, resulting in the axillary position of sporangia in the lower groups, as well as in the ‘Schuppenkomplex’ of the female Coniferous cone and axillary stamens and ovules (or placentae) in stachyosporous Angiosperms. MPB Units are characteristic of stachyosporous groups and relatively efficient. In these groups overtopping was an early prevailing principle in the reproductory sphere; in the vegetative one (main ramification) dichotomy was preserved in some groups (Psilopsida, Lycopsida). 12. 12.Similar units, though not protective, are found in the Ophioglossales as well as in such lepto-ferns as Aneimia. They are here called Mixed Bifurcation Units(MB Units). They are the basic type in the phyllosporous groups, in which the dichotomies were (and are) long preserved, both in the vegetative and in the reproductory spheres; overtopping is acquired much later and mainly in the vegetative sphere. The intrinsic protective power of the MB Units is weak and had to be improved by several auxiliary structures (cf. point 14). 13. 13.The association of leaf and axillary shoot is based on the same principle as is valid for the MPB Units. In the first-named case we may speak of Sterile Protective Bifurcation Units (SPB Units). They confirm the interpretation of the axillary flower as a modified shoot. It is conceivable that in both types correlative processes have developed, by which the two components influence each other physiologically. The latter process is supposed to occur in the cones of both sexes in Cycadaceae, and the male ones in Coniferae, in which the sporophylls are not known to produce axillary buds, sterile and fertile being known to be antagonistic. In younger Sphenopsida a diagonal SPB Unit is found between leaves and branches, based on the peculiar venation of the node. 14. 14.Other types of protection were acquired in the phyllosporous ferns, such as position of the sporangia on the lower leaf side (except where the leaf is infolded, as is the case in Marsilia), forming of thick-walled synangia (eusporangiate ferns) and indusia (leptosporangiate ferns). 15. 15.It is stated that superprotection of megasporangia was first obtained at an early period by the development of one or more integuments and that these integuments have originated by the shifting upwards of sterile telomes or syntelomes, often two of them, as is intimated by the venation of several integuments. 16. 16.Two types of integument are recognised: the type of the Pteridosperms and the Cycadales with dichotomising nerves, and the type of the Coniferopsida, Cycadeoidales, etc., which is bivalved and mostly not vascularised. In several ways, Ginkgo takes an intermediate position. 17. 17.There seems to be no or little correlation between the two integument types on the one hand, and stachyospory-phyllospory on the other. 18. 18.A megasporangium covered by an integument is called an ovule. It is suggested that integuments belong to the same morphological category as the cupule of the Pteridospermales, arilli in many groups and, in some cases, pseudocarpels. 19. 19.In some Ephedra species the pairs of decussate leaves on the axillary female shoot gradually merge into what has sometimes been called perianth, sometimes outer integument. It is suggested that the series is continued up to and including the innermost integument. 20. 20.It is further supposed that the same condition is to be found in some Monochlamydeous Angiosperms with ‘false carpels’ (pseudocarpels) in which case another distinction between Gymnosperms and Angiosperms is fading away, the only remaining ones being the double fertilisation and the ensuing secondary endosperm, characters which, like the others, may prove to be of doubtful differential value. The concept ‘flower’, having no objective morphological status, is accordingly differentially useless and should not be used as a botanical term (cf. point 8). 21. 21.Under these circumstances Casuarina had better be reinserted in the Angiosperms and accordingly the Protangiosperms as a class dropped. 22. 22.Superprotective structures are of several kinds and levels: the supercupula of Calathospermum, peltate and closely congregated sporophylls in the Zamiaceae, close aggregation of ovules and interspersed sterile organs in the Cycadeoidales, of ovules in the Pentoxylales, of Mixed Protective Bifurcation Units in the female coniferous cones, Chlamydospermales and stachyosporous Angiosperms, infolding of sporophylls in phyllosporous ones, inferior ovary in both, aggregation of flowers into inflorescences and of these into superinflorescences. 23. 23.Sporangia of both sexes have, like the sterile telomes, retained their original autonomy in whatever way they are secondarily grouped, arranged, or protected. 24. 24.With the progressive evolution of the sporangia, the regressive one of the gametophytes goes hand in hand. The peculiar role played by the ventral canal-cell is described and the possible pursuance of the line of evolution given in outline. 25. 25.Interchangeability of sterile and fertile ‘telomes’, as well as stem and leaf, is to be expected even in the Angiosperms. It may account for several controversies in the interpretation of organs as well as for that of certain terata. 26. 26.The autonomy of such organs as stems and leaves is mostly overrated. The existence of organs of an intermediate origin should be seriously considered. 27. 27.In endeavouring to interpret present organs and their phylogenetical relations we have mostly to dig much deeper than was originally anticipated. 28. 28.The concept of homology (cf. point 7) is discussed again. It is stated that recently attempts have been made to give it ontogenetical implications. Recent investigation in the biochemistry of meristems seem promising in this respect. 29. 29.Anatomical, ontogenetical, and in general static (one-time) data alone can never be conclusive towards the solution of phylogenetic problems, though theirevidence may be of outstanding importance when interpreted in a phylogenetic (dynamic) sense.