DORSI-VENTRAL AND BILATERAL STRUCTURE. 489 



such enveloping leaves, which alternate however with the preceding leaves 6-10, 

 i.e. fall in the interspaces between their five radii : finally, the tripartite figure in the 

 centre of the flower £ is the ovary composed of three leaves. It is thus seen that the 

 arrangements of the leaves on one and the same axis changes in this as in most 

 other cases ; moreover, the arrangement of the leaves on the five lateral shoots, as 

 the diagram shows at once, is again different from that on the main shoot. In 

 spite of all fhese irregularities, however, the radial type is nevertheless maintained, 

 at least on the main shoot of the plant ; the outgrowths radiate in four or five 

 directions,- and there would be no essential difference if they radiated in eight or 

 thirteen directions. These very few examples will probably suffice to show the 

 reader what is implied by the term radial. 



Radial structure stan ds in di rect contrast to bilateral structure, and here we 

 have at once to distinguish two different cases, namely, ordinary bilaterality', and the 

 bilaterality which is combined with dorsi- ventral oiiganisation. If, for example, we have 

 an upright shoot furnished with two opposite rows of leaves, and we suppose this so 

 bisected longitudinally that all the leaves are at the same time also bisected, the two 

 halves of the shoot appear nearly symmetrically alike. If, on the contrary, we make 

 the plane of division at right angles to the previous one, so that the leaves are not 

 divided, we again obtain in the same way two similar halves, which are now however 

 conditioned diff'erently from the halves of the first division. In the first case each 

 half of the shoot has two rows of half leaves ; in the second case each has one row of 

 whole leaves. Such a structure might just as well be named quadrilateral as bilateral, 

 or even doubly bilateral. In any case, a symmetrically similar second half exactly 

 corresponds to each half, and when gravitation, light, and other directive forces act 

 equally from all sides on such a shoot it places itself vertically — it is orthotropic, like a 

 radially constructed shoot, and it is physiologically to be relegated to the radial type. 

 / Much more frequent and essentially different from the latter, however, is the 

 dorsi-ventral bilaterality met with in very many creeping and climbing shoots, in all 

 leaves, and even in very many lateral shoots which spring from orthotropic and 

 radial main shoots. The dorsi-ventral organisation has always as a consequence 

 that the organs concerned are plagiotropic towards external influences, i. e. the effect 

 of gravitation, light, and other directive forces induces such organs (which are 

 usually moreover extended flat) to place themselves across the direction of gravitation 

 and of the ray of light. We have to regard as the commonest examples of dorsi- 

 ventral bilaterality the ordinary flat foliage-leaves. Their bilaterality is at once clear 

 on observing how two halves of the lamina exist right and left of the mid-rib ; these 

 are usually nearly symmetrical in leaves on upright shoots ; in those on horizontal 

 or oblique shoots they are generally more or less unsymmetrical. The leaves of the 

 Elm and the so-called obhque leaves (e. g. Begonia) afford well-known examples of 



' The doubly bilateral but not dorsi-ventral organs are closely connected with radial organs, 

 both as respects geometric considerations and their reactions towards external influences ; in par- 

 ticular they are, like those, orthotropic, as is easily perceived in the case of the very excellent 

 example of the gemms of Marchantia. Hence, -although from a purely formal and geometrical 

 point of view, the chief contrast appears to lie between radial and bilateral organs ; nevertheless, so 

 far as physiology is concerned, the contrast between radial and dorsi-ventral organs is much more 

 important. 



