Investigations of the distribution of growth in normal and experimentally treated apices of Dryopteris aristata Druce, and of factors which determine the form and structure of leaves, buds and scales, are described and discussed. The conclusion is reached that some of the classical views on the organization of the leafy shoot are not compatible with the data now available. Growth is considerably more rapid in the subapical region than at the apex. Leaf primordia grow actively, but growth is slow in their axils and in certain interfoliar positions. As a result a system of stresses is induced in the meristem; this may be a factor determining the positions of new leaf primordia. Shoots from which all primordia, leaves and roots had been removed became attenuated, but leaf primordia and scales continued to be formed at approximately the normal rate. The primary morphogenetic processes at the apex are thus apparently independent of the presence of primordia, leaves and roots. When the apical meristem was isolated by deep vertical incisions it grew slowly and decreased in size; but lateral buds formed in close proximity soon developed to large size. When the apical cell was damaged, the growth of the isolated terminal region was carried on by one or more buds which developed from cells of the meristem. Apical meristems, three-quarters ringed, with concomitant severing of the incipient vascular tissue, developed symmetrically; hence it appears that the upward movement of nutrients to the meristem takes place over the whole cross-sectional area of the shoot and not specifically by way of the incipient vascular tissue. When incisions were made very close to the apical meristem a protostelic shoot, with a solid core of tracheides, was formed, i.e. the dictyostele was reduced to a protostele by a single operation. In many of the large buds which developed in the subapical region a solenostelic vascular system was differentiated, there being no antecedent protostelic stage as in the normal development. This finding is of interest in relation to classical views on the nature and evolution of the fern stele. When the apical cell was punctured, leaf primordia continued to be formed, usually in normal phyllotactic sequence, until all the space on the apex had been utilized. Bud primordia were formed in the subapical region and lower part of the apical meristem, their formation being subsequent to that of the leaf primordia. Scales developed on the meristem round the margin of the necrosed tissue. The formation of leaf primordia is thus independent (a) of the apical cell, although the presence of the latter is necessary for the continued growth of the meristem, and (b) of older primordia and leaves, though the possibility that these members may exercise an indirect effect is not excluded. Buds are not inhibited by leaf primordia, but they are by substances proceeding from the apical cell and possibly its immediate segments. The view is advanced that the chief differences between leaf and bud primordia are referable to the positions which they occupy at the time of their formation. A re-examination of leaf formation has shown that the very young primordium originates not from a single superficial meristematic cell, as described in the literature, but from a group of meristematic cells. At an early stage, however, one of the more centrally placed cells begins to enlarge and becomes the conspicuous apical cell of the primordium. The underlying cells also undergo rapid division. These data have a particular interest in that they show that leaf formation in ferns and flowering plants is in the main essentials closely comparable. Leaf primordia of different ages, isolated by vertical incisions, undergo some further growth: the older primordia may form pinnae but the younger ones terminate in a straight or outwardly-curved, awl-like structure; in the latter, the apex loses its meristematic character and becomes parenchymatous and the vascular system gradually fades out. According to the size of the primordium at the time of isolation, a foliar gap may or may not develop in the shoot stele; the leaf trace may develop as a solenostele, it may remain coherent and crescentic in cross-section, or it may become disrupted into meristeles. In a majority of isolated leaf primordia, axillary buds develop and become the predominant feature; these buds may be solenostelic from the outset; lateral and abaxial buds may also occur. Even when very young leaf primordia are isolated, the typical dorsiventral symmetry, which is evidently established at a very early stage, is retained. In the formation of plant organs, factors in the genetic constitution, physical factors of various kinds, and factors in the environment are all involved. In practice, these are difficult to distinguish. From the writer's analysis it appears that many of the major morphological features of vascular plants are to be related to extrinsic rather than to genetic factors; the two kinds of factor, of course, work in conjunction. It is a matter of general interest to botanists to know which aspects of morphological development are due primarily to extrinsic factors and which to intrinsic or genetic factors; in investigations into causality our primary concern is with the former, in phylogenetic studies it is with the latter. Reasons are given for the view that the shoot type of organization in different classes of vascular plants is probably the result of parallel evolution, i.e. homoplastic development. Furthermore, it is suggested that the lateral members are not necessarily determined by specific factors in the hereditary constitution. Thus, in a plant with the potentiality for producing lateral members, the form and structure of a lateral organ, be it leaf, bud or scale, depend primarily on the position in which it is formed at the apex; i.e. given a certain specific hereditary constitution, it is to the mechanics of growth in the plastic distal region of the shoot that we must look for explanations of the characteristic symmetry, form and structure of the several lateral organs. But ultimately, in every investigation of morphogenetic processes, the hereditary constitution of the organism must be considered and an attempt made to understand how the several kinds of factors, by their action and interaction, determine specific form and structure.