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Transcribed, by permission, from Tuatara (1984) Vol. 27 (1): 26-48 Principia Botanica: By Michael heads INTERPRETATION OF THE ROOT The question of the seed plant's root has been answered even less satisfactorily by descriptive morphology than has been the question of the leaf. Descriptive criteria fail dramatically when faced, for example, by Lentibulariaceae, and in fact, there are virtually not concepts efficient in producing even minimum of understanding. In Chapter 11 of the Principia Botanica Croizat begins his discussion of the root with an analysis of the structures found at the junction of hypocotyls in the seedlings of various taxa. These structures (known in the literature as "wurzelhals", "foot", "peg", "collet", etc.) are much more widely distributed throughout angiosperms than was formerly realized, occurring in such taxa as Hippuris, Curcurbitacae, Triglochin, etc. In Eucalyptus erythrocoris the normally horizontal collet forms a sheathing structure morphogenetically identical to the graminaceous coleorhiza. In the embryo of seed plants, root and plumule, representing contrasting polarized centers of development are joined by a transitional zone, the hypocotyls. Beyond the root intials lies the meristem (e.g. the rib meristem of Pseudostuga Allen 1946) from which arises the root cap. These three embryonal meristems are all bound within a jacket of ground tissue, Croizat's synthetic concept which comprises a sheath of considerable morphogenetic powers. The jacket is responsible for the development of various structures, for example tubers, napiform taproots, the collet, the haustorium of parasitic plants, the holdfasts and coralloid roots of the Podostemonaceae, the grass coleorhiza, and of course the rather inconspicuous cortical layer surround the hypocotyls in other "higher" plants. The concept of "jacket" is an efficient tool for rationalizing many aspects of root morphology from Pseudotsuga to Triticum, to Utricularia. But a fundamental problem is offered by such plants as Podostemon ceratophyllum which have no distinct root, but do possess a root cap. In fact Croizat recognizes the problem of root cap as of fundamental significance. He suggests that it is advisable to maintain a general concept of rhizophore for what is currently terms "root", "rhizome", "runner", "hypocotyls", "pneumatophore" etc. in opposition to one of root cap. Thus the rhizophore is seen essentially as the bearer of the root cap, and structures such as the underground axis/rhizophore plus roots of the form genus Stigmaria, and the corm (rhizophore) plus roots of Isoetes, long considered problematical, are simply interpreted as relatively undeveloped morphologies in the morphogeny leading to angiosperm roots. (c.f. Stewart 1947; Sporne 1975: 69-70). Croizat's analysis has not been developed by modern workers on theroot, but it is clear that this is the least understood plant organ of all, and basic morphogenetic concepts are urgently needed. Too many recent studies of root form still provide "explanations" of that form in term son of function and adaptation! Invariably aspects of root morphology apparently restricted to a small number of taxa are regarded as "advanced", "derived" etc., homologized with something or other, and forgotten about. A good example is provided by the "macrodpodous" condition in certain monocotyledonous embryos (see Dahlegren and Clifford 1982: Fig. 8). OTHER BOTANICAL CONTRIBUTIONS This review has so far mentioned some of Croizat's work on the fundamental of botanical philosophy, but it would be unwise to ignore altogether his contributions to other aspects of botany. Croizat felt it was crucial to be able to place work and ideas in their historical context. In the Principia Botanica he seldom considered this as an end in itself, but the importance he placed on it and the interest it held for him can be seen in various other publications dealing largely with historical questions (1945) and biography (1949a, b). Most of Croizat's early work deals largely with the taxonomy and nomenclature of Cactaceae and Euphorbiaceae, and is of only minor interest to the botanist who does not have a special interest in and knowledge of these families. In much of his work it is interesting to observe the enthusiasm with which Croizat deal with the horticultural aspects of his favored plants (e.g. 1941a). Practical rather than theoretical question provided the impetus for all of Croizat's work which was not done in a vacuum but is of interest to all laymen and scholars who ask the question "Why is this plant the way it is, where it is?" It goes almost without saying that Croizat was a formidable enemy of ivory-tower scholasticism. Because of the practical necessity of supplying a name for a plant, Croizat was naturally concerned with the correct application of the rules of botanical nomenclatures, and also took an active interest in the development and improvement of the rules themselves (e.g. 1941b, 1953). Croizat has published several large and important botanical works since the Principia. In Croizat (1964a, b) his ideas on angiosperm evolution in space, time, and form are summarized and, to an extent, developed and refined. Croizat (1965, 1967, 1972a, 1973a) provide both an overview and a detailed analysis of Euphorbiaceae, Euphorbieae and Euphorbia and represent conclusions reached after several decades of cultivation and study of these plants. Croizat's work on these taxa is undoubtedly among the most important yet produced. Croizat (1970) is a critical interpretation of Corner's fascinating but almost totally neglected Durian theory of the origin of the modern tree. Croizat (1971, 1972b, 1973b) are very important contributions to the study of the leaf and of phyllotaxy, in which a considerable amount of mew material is introduced. Although the most striking aspect of the Principia Botanica is undoubtedly the fundamental nature and originality of the principles developed, the reviewer would be failing in his task if he did not attempt to place Croizat's botany in some sort of historical perspective. As with his biogeography, many of his ideas were hinted at, though scarcely developed by earlier students. Cusset (1982), in which must be regarded as a landmark of botanical historiography, has produced a reviewer of the conceptual bases of plant morphology. In it the affinity of Croizat's concepts, particularly concerning foliation, with those of botanists such as Warming and Trécul is underscored. In the fields of floral morphogeny and high systematics Croizat's conclusions often reflect more his sympathy with those of Baillon, a botanist neglected by modern students. Many of the principles established in Croizat's work are becoming accepted, or tat least discussed, by the botanical community, but too often the work is cited only by leading researchers examining fundamentals of botanical knowledge. Obviously even the reading of a large work such as the principia is a major undertaking much time is required to digest the arguments and check up on examples in field, herbarium and literature. But probably the main reason for the surprisingly small number of papers discussing Croizat's ideas is the fundamentally heterodox nature and the complexity of the analyses. As his ideas become more acceptable to the botanical community more credit will undoubtedly be given Croizat for the great significance of his contributions to the "Beginnings of Botany".
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