MORPHOLOGY OF PLANTS The morphology of plants consists in their comparative study from the point of view of Form. It treats not only of the external contour of the plant-body, External Morphology; but also of the internal contours of the constituent tissues, as revealed by ana tomical study, the Morphology of Tissues. For the full under standing of form it does not suffice to examine the adult state only. It was found early that difficult questions of comparison could be illuminated by observation of development of the indi vidual, that is, of its Ontogeny. This led to views based upon com parison as to the historical origin and descent of the parts com pared, or even of the plant as a whole. A further step to the study of the parts as organs, carrying out certain distinct functions, gives the aspect now designated as Organography. These methods of enquiry are used collectively as a basis for evolutionary or sys tematic conclusions. But such conclusions are apt in critical cases to take the form of expressions of opinion which may differ widely. In arriving at a decision the most positive source of knowledge is the evidence of geological succession in time, and comparison of the forms constituting that succession. This is yielded by the study of fossil plants, called Palaeophytology or Palaeobotany (q.v.). But its record is so incomplete as to leave many problems unsolved. The morphology of plants thus conceived bears within it the possibility of reconstructing the evolutionary history of the vegetable kingdom, which is the goal to which all such study con verges. This is called Phylogeny.
Pre-evolutionary and Evolutionary Morphology.—The aspect of morphology thus sketched is modern, dating in its pres ent form from the period of the Origin of Species. But with vary ing success pioneer efforts had been made to found a science of plant-form before 1859, when Darwin's book was published. At first these related to the parts that constitute the plant-body as seen in the higher flowering plants. The foundation of organog raphy was traced to Aristotle by Vines. More recently classifica tion of members of the plant-body by Jung (1657), and observa tions on their ontogeny by Wolff (1759) resulted in a view sum marized by the latter in these remarkable words : "In the entire plant whose parts we wonder at as being, at the first glance, so extraordinarily diverse, I finally perceive and recognize nothing beyond leaves and stem, for the root may be regarded as a stem.
Consequently all parts of the plant, except the stem, are modi fied leaves." With such a conclusion already before him it was an easy step for Goethe (179o) to enunciate his theory of Metamor phosis of Plants : in particular he visualized an ideal leaf of the nature of a foliage leaf, of which the bracts and floral parts were held to be metamorphosed types.
The theory of evolution, long entertained before but galvanized into active existence by the genius of Darwin, revolutionized mor phology (Origin of Species, 1859). For those whose vision was clear it converted it at a single stroke from a deductive handling of abstract ideas based chiefly on the higher plants, to an inductive argument rising, as Bacon said, successively from particulars to axioms of the lowest generality, then to intermediate axioms, and so to the highest.
This newer evolutionary or phyletic morphology, better desig nated as organography, starts with simple unicellular and aquatic organisms, such as the Flagellates. Each is a minute naked proto plast of inconstant form. A first step towards definite plant organization is encystment within a firm cell-wall: this state appears as an incident in the life of many Flagellates, the encysted cell taking an approximately spherical form. Growth of such a green cell gives better opportunity for nutrition, and for storage of the material acquired : and an enlarging spherical form may be retained by organisms that float in the uniform medium of water. But the relation of an enlarging body to external conditions not uniform is liable to result in polarity, with the fundamental distinc tion of apex and base. The latter commonly attaches itself to any solid substratum, even without division of the single cell. But usually cell-division follows upon increasing size. If polarity be strongly marked, and growth continued, a filamentous plant-body may result, consisting of a chain of cells attached at its base. This may be further elaborated by cleavage of cells and by branch ing, while still attached at its base. In this we see initiated another leading characteristic of plant-organization, viz., fixity of position.