In two studies, the first in a school in Peterborough and the second in a school in Norwich, more than 233 children aged 8-12 years received either an orthoptic examination, or an optometric examination, together with an examination using coloured overlays and a test of reading fluency. In both studies more than one-third of the children reported visual symptoms. More than one-third of the children chose to use an overlay, and they read more quickly with it than without. The colour of the overlay chosen was weakly related to the binocular amplitude of accommodation: overlays reflecting greater energy at long wavelengths were chosen more frequently by children with a higher amplitude of accommodation. Although the visual symptoms were strongly related to the use of an overlay, in neither study was the benefit from an overlay strongly related to the orthoptic or optometric findings. Nevertheless, children who used an overlay had slightly, but significantly, reduced mean binocular amplitude of accommodation and fusional reserves. On average, children with 'sensory' or 'motor' instability of the nonius strips of the Mallett unit read more slowly than others, as did those with poor stereopsis. However, 60% of those demonstrating sustained overlay use gave a normal response on the Mallett aligning prism test, compared with 80% of those who did not use an overlay for a sustained period. Another indicator of decompensated heterophoria, Sheard's criterion, did not differentiate subjects who used overlays from those who did not. Although binocular and accommodative anomalies do not appear to be the underlying mechanism for the benefit from coloured filters in most cases, there may be some individuals who respond to coloured filters and in whom these ocular motor factors require treatment. Children with visually precipitated symptoms and/or reading difficulties need both a careful evaluation of their accommodative and binocular status, and an investigation of the effect of coloured filters.

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Summary. The problems of phyllotaxis are re‐examined from the standpoint of the initial adjustment of the units of shoot growth. The structure of the shoot apex is reviewed in relation to the origin of leaf primordia and it is pointed out that such primordia, once formed, are competing centres of activity and consequently tend to arise as nearly as possible opposite to one another, as exemplified by the simplest case of alternate or 1/2 phyllotaxis. When more than two primordia are growing simultaneously at the apex, successive ones cannot be exactly opposite, as in this case the position of the new primordium is influenced by all the others which are growing at the same time. By application of this conception of competing growth centres, it is shown that, as the number of growing primordia at the apex rises, the fraction denoting the new type of phyllotaxis is likely to fall in the Fibonacci series, because analysis shows that these are the only ones which satisfy the requisite spacing of the primordia in relation to the number of time intervals or “plastochrones” separating them in origin. It is pointed out that systems belonging to the higher fractions in the Fibonacci series are practically impossible to recognise with certainty, unless external observations are supported by anatomy. The case of Iberis amara is analysed in detail and it is shown that the 5/13 system could be derived from anatomy. A rise in the number of primordia growing simultaneously involves readjustment of the units of shoot growth and it is shown that, as the system rises from one fraction to the next, characteristic displacements take place, which vary in direction in different transitions. The higher systems are usually associated with a wider pith. It is concluded that the direction of the spiral is determined by the positions of the two first primordia, the genetic spiral being merely an abstraction from the developmental standpoint. This is illustrated by the fact that the direction of the spiral on branches seems to vary irrespective of the spiral on the axis of the same plant. A decussate system with an angular divergence of 1/2 between members of a pair and 1/4 between successive pairs is regarded as a stable system, provided that the primordia of a pair arise almost simultaneously and time intervals between the pairs lengthen proportionately. This argument is analysed from measurements of the plastochrones in whorled and spiral plants of the same species. Although decussate types often show slight differences in the time and position of development of the members of a pair, this merely demonstrates the tendency to successive emergence of primordia and not the evolutionary derivation of the system from a theoretical spiral system. If a plant has a tendency to decussate phyllotaxis, this system naturally appears in the seedling because of the temporary halt in the embryonic growth at a stage when two cotyledons occupy the apex. In axillary branches, the symmetry of the phyllotaxis is affected by the subtending leaf and often start...

Summary. The investigation was started in the hopes of throwing some light upon the problem of sap supply to the upper parts of leaves which grow by a basal meristem, but has been extended as a developmental study of the shoot. The scattered bundles of the stem may be classified into four groups–(a) medullary, (b) perimedullary, (c) cortical, (d) peripheral. At the nodes the bundles of the different series are in connection. up the second internode as the perimedullary bundles which constitute the leaf‐trace. The differentiation of the protoxylem of the primary longitudinal veins of the leaf takes place basifugally, but when this differentiation reaches the leaf‐apex, the direction of sap‐flow is reversed, with the result that the secondary longitudinal and transverse veins develop basipetally in the leaf, and lead to the basipetal development of the cortical and peripheral stem‐bundles. The internode grows by an intercalary basal meristem–the disappearance of which may be traced in a basipetal direction by the gradation in cell size and the micro‐chemical reactions of the cell‐walls in an elongating internode. The peripheral bundles develop in the region of the peripheral meristem, and in the course of development become surrounded by an endodermis, which is later converted into a sclerenchymatous bundle‐sheath, continuous with the sclorenchyma sheath running round the stem. The leaf primordium consists of an incomplete ring of tissue which later differentiates into sheath and lamna. The differentiation of the leaftissues follows the same general plan as the vascular development, taking place chiefly in a basipetal direction. The effect of etiolation is to produce relatively narrower leaves. The following points are discussed as being of importance in the development of the shoot in Tradescantia, and as probably bearing upon the Monocotyledon in general:– The absence of secondary meristems and consequent importance of the portions of the primary meristem, isolated as intcrcalary stem and leaf meristems. The zonal upgrowth of a few leaves from intercalary rings of meristom. The reversal in the direction of sap‐flow from the apex of the lamina with its effect upon the irrigation of the basal meristem of leaf and internode, the formation of leaf auricles, and finally in the development of the peripheral stem‐bundles as alternative channels of vascular supply to younger parts of the shoot when the older channels are ruptured by clongation of the internode. The zonal distribution of meristem accounts for the continued growth of partly developed leaves under etiolation conditions. No new leaf initials separate from the apex during growth in darkness. It is shown that the change in shape of the leaves of Alisma, when grown in darkness, might be anticipated from a considoration of the developmental conditions under which the expanded lamina is formed.