Text-book of botany, morphological and physiological . angles,the angles corresponding to the ortho-stichies of the spirally arranged leaves,which, however, in this case mostly re-main undeveloped. In verticillate leavesalso the straight rows are mostly conspicuous if the shoot is looked at from above,as, for instance, in the decussate two-leaved whorls oi Etiphorbia Lathyris, and thecactus-like E. canarieiisis. When the members of a spiral phyllotaxis with a constant angle of divergencestand sufficiently close to one another, spiral arrangements are easily seen andfollowed to the right and le
Text-book of botany, morphological and physiological . angles,the angles corresponding to the ortho-stichies of the spirally arranged leaves,which, however, in this case mostly re-main undeveloped. In verticillate leavesalso the straight rows are mostly conspicuous if the shoot is looked at from above,as, for instance, in the decussate two-leaved whorls oi Etiphorbia Lathyris, and thecactus-like E. canarieiisis. When the members of a spiral phyllotaxis with a constant angle of divergencestand sufficiently close to one another, spiral arrangements are easily seen andfollowed to the right and left which more or less conceal the genetic spiral. Theserows are called Parastichies, and are particularly clear in the cones of speciesof Pinus, the leaf-rosettes of Crassulaceae, the flowers of the sunflower and otherCompo^itee, and in the spadices of Aroide^. They may be seen in every spiralphyllotaxis with a constant divergence, and can always be made clear in the dia-gram, or when the arrangement is represented on an unrolled cylindrical Fig. 145.—Diagram of a shoot in which the leaves have a cuiistant phyllotaxis of tj. 1/4 EXTERNAL CONFORMATION OF PLANTS. The considerations of these constructions leads to definite geometrical rules, bymeans of which the genetic spiral can be easily deduced from the parastichies \It is evident that the constructions hitherto mentioned can only be more orless convenient aids to an understanding of the actual principles of the arrangementof leaves. But in order to obtain, with their assistance, a deeper insight into theprocesses of grow^th themselves of which these principles are the result, it isnecessary to follow up the history of development, and in every single case toask the question, what circumstances are the cause of a new member being formedjust in this place and nowhere else. It may be well, therefore, to bring forwardhere some of the points which must be considered in reference to this view. (r) The first question to
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Keywords: ., bookcentury1800, bookdecade1870, booksubjectbotany, bookyear1875
