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The Principles of Morphology.—I 124 ' ^- C. Worsdell. pure cultures. These would increase in numbers and complexity as the science develops, and we might then expect to be in a position to investigate bacterial processes, as they occur in nature, more fairly and with greater success. HARHIETTE CHICK. THE PRINCIPLES OF MORPHOLOGY.—I. INTRODUCTION. IT is by a study of the evolution of the forms of life composing the vegetable kingdom that the science of morphology has been gradually built up. It is a study exceedingly important in itself, and quite distinct from that of physiology, which deals solely with the functions which those various structures perform. The principles of morphology are the inherent and essential laws governing the production of form in the vegetable world, which we deduce by means of a study of the phylogenetic sequence' (or what we regard as such) of forms through family after family of plants. As regards the external conformation of the typical plant, I believe myself that an exceedingly limited number of fundamentally discrete and individually distinctive organs (organs sui generis) occur which have been laid down as such from nearly the commencement of the plant's evolution, viz. the caulome, phyllome, and rhizome,'' for it seems quite possible to trace these under their innumerable and infinitely varying forms from the earliest and most primitive types onward to the very highest; and not only that, but also along all the distinct phyla into which the Vegetable Kingdom has become split. And hence, to take an illustration, we should expect homologous organs to occur, in each of the great phyla, Lycopods, Equisetaceas, Ferns and Flowering Plants, at every stage in the individual life-history of the plant. The " morphology " of any doubtful organ would hence be often arrived at by means of a close comparative study of related organs (or what we regard as such) throughout the entire range of vascular plants. This comparison must necessarily be exceedingly fundamental and • It is just here where, in the absence of perfect objective continuity oi the forms concerned, the subjective interpretation of such facts as are available, plays so predominant a part. ' This word is here used in its literal, original sense, not in that of our text-books. The Principles of Morphology. 125 uninfluenced by prejudiced conceptions of the phylogeny of any particular group. I believe also that teratological phenomena are in this direction very helpful, for in these strictly-limited mani- festations of morphological law, we find the organs assuming forms, the replicas of which will be found for the searching in other departments of vegetable life. The facts of individual development will also sometimes be of use, but very often also misleading, on account of the fact that many of the recapitulative stages derived from the race-history are missed out owing to the abbreviated course taken by the ontogeny. It is, then, owing to the stereotyped fixity of the three categories of organs, whereby no obliteration of the distinctive boundaries of each, i.e. no fusion or confusion of any two categories, ever takes place, that a definite science of morphology becomes possible. Each type of organ, whether caulome, phyllome or rhizome, has evolved, through infinite variation, along its own line, diverging away from each of the other types, precisely in the same way as genera and species of plants have evolved into distinctive entities. In any particular case, where it is especially difficult or impossible to determine the morphological nature of an organ, this must be attributed solely to our own ignorance and inability to trace all the stages in modification which the particular organ in question has undergone during the period of its evolution. THE OUIGIN OF THE VASCULAR PLANT. I. THE ALTERNATION OF GENERATIONS. HE vegetable kingdom, doubtless, was at one time constituted T solely of unicellular organisms which eventually gave rise by successive cell-divisions both to the filamentous and lamelliform types. Filamentous aggregations, forming so-called individual plants, were produced by branching of these filaments; yet multi- plication of the thallus also eventually occurred by means of tissue- formation, either in the form of a true tissue, as in Liverworts and Mosses, or of a false tissue, such as that of Seaweeds and Fungi. The thallus of these early plant-types assumes various forms; it is either dorsiventral, as in Liverworts; cylindrical, asintheMushroom- stalk and prothallus of some Lycopods; partly cylindrical and partly isobilateral, as in Brown Seaweeds, or filamentous, as in many Algte, Fungi, Prothalli, and the Moss-protonema. 126 W.C. Worsdell. The most primitive unicellular plants were almost certainly asexual, the sexual character being a subsequent modification ; for the undifferentiated must precede the differentiated along the main path of evolution; and the very earliest and most important form which differentiation assumes is the production of the two opposite yet complementary poles: the positive and negative, or male and female Sex, by the blending of which the greatest amount of vitality and capacity for variation is infused into the product. Now, the asexual unicellular forms most probably have given rise to asexual filamentous organisms, and these must have been the earliest progenitors of all filamentous Algae' with which we are acquainted to-day. Sooner or later, however, sex also arose in these filamentous and other thalloid forms; it is a noticeable fact that in thallus-forming plants both sexes occur as a rule on the same plant. There can be no doubt that the spermatozoid and oogonial cell of certain more advanced filamentous Algae, e.g. Vaucheria, are the exact homologues of the free-swimming male and female gametes of other more primitive forms, such as Ulothrix. In any case, in the primitive forms the result of fertilisation was (at any rate frequently), the germination of the zygospore directly into a new filamentous plant which was asexual, inasmuch as it did not produce sexual organs but, on the contrary, neutral zoospores. Each of these latter, on germination, produced the sexual plant once again. Here we see a distinct alternation of generations: but it is an homologous alternation of generations, viz., between a neutral or gonidial and a sexual or oogonial generation respectively ; the two generations are homologous because both exhibit the same mode and habit of growth in every respect, they are precisely the same plants differing only in the character of their reproductive organs. Examples are afforded by Ulothrix, Vaueheria, Dictyota, &c. amongst the Algas. A very interesting example of this method of alternation is found in the Moss, where the primitive asexual filamentous protonema gives rise (herein exhibiting a striking adaptation to its acquired land-habitat) by vegetative budding, and no longer by zoospores, to the leafy Moss-plant.- Yet another instance of this homologous alternation of generations is to be observed in the higher Fungi and Florideae; in the former the ascus and basidium and in the latter the carpo- ' 1 use this term to include also all primitive Fungi. ' Here the two generations differ altogether in habit and mode of growth ; nevertheless they are clearly equivalent to the two generations constituting the homologous alternation in the Algs. The Principles of Morphology. 127 spore, are to be regarded as the equivalents of the archegonium and oogonium in ATuscineae and Alg£C. In the two latter groups the investment or wall of the female organ is formed before fertilisation ; in the former groups (Fungi and Florideae) it is formed subsequent to that process, this being analogous to the varying periods of formation of the endosperm in Gymnosperms and Angiosperms. There exists no sufficient difference between the two generations here produced to render it other than a case of homologous alter- nation of generations in these plants. This probably represented the cycle of life for many an age. The fact that, as in Vaucheria, the two generations blend at certain points, or that, as Klebs has shevvn, either can be produced at will by modifying the immediateconditionsof life,does notaffectthe reality of the presence of the above-described alternation of generations, for these secondary modifications are justwhatmightbe expected to occur in such plastic organisms as these primitive Algte. inOLdogoniitm the neutral generation, as an independent stage, has been cut out of the life-cycle, and the oospore divides directly into four zoospores, each of which reproduces the sexual plant. This zoospore-for- mation in the oospore represents the first faint fore-shadowing of an entirely new generation in the life-cycle, viz., that which Celakovsky has termed the "antiphyte" or the "antithetic" generation. In Coleoeha:te this suggestion of the new generation is to hand in plainer terms, for the contents of the oospore divide up into a small parenchymatous tissue from each cell of which a zoospore is rounded off which, escaping, tbe empty cell-frame-work is left behind, thus differing from CEdogonium in which each entire cell itself becomes a zoospore. Yet in Coleochcete the actuality of a new antithetic generation is not attained; the complete absence of a limiting wall of sterile cells debarring this structure from attaining to the rank of anything resembling a unity in itself. This distinction has been reserved for Rieeia, the most primitive of the Bryophytes, in which we discover the first appearance, in its lowest terms of simplicity, of the antithetic sporophyte generation.' In the higher Liverworts and in the Mosses this sporangium became much more elaborated, and became, although remaining attached to the thallus, a distinct plant of an entirely new type. For, in the first place, the mode of growth, i.e. the method of tissue-formation and ' Hence the above-described structures in CEdogonium and Coleochcete represent the actual and real progenitors of the antiphytic generation which is, therefore, very far from arising ex nihilo or from being a "morphological Melchisedec," as Scott so strangely imagines.
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