1/6 EXTERNAL CONFORMATION OF PLANTS. 



(5) It must further be observed whether the first appearance of lateral mem- 

 bers is preceded by phenomena connected with development which assist in the 

 determination of the place of origin. Of this nature, for instance, is the connexion 

 of the points of origin of the lateral roots on the outside of the fibro-vascular 

 bundles by the course of which their succession is determined ; and this again, in 

 its turn, determines whether the lateral roots are arranged spirally or in whorls. 

 Here the arrangement in longitudinal rows is clearly the general and prim.ary one ; 

 the divergences and longitudinal distances are a secondary affair determined by 

 special accessory circumstances. The point of origin of a lateral shoot is, on the 

 other hand, in general primarily determined by its relation to the nearest leaf, in 

 so far as it is formed beneath, beside, or above its median plane ; forces of secondary 

 importance determine whether lateral shoots are formed in connexion with each 

 leaf or only with particular leaves of an axis, and so forth. The phyllotaxis of the 

 lateral shoot may differ from that of its primary shoot, because the growth of the 

 latter assists in influencing it ; as, for instance, in the case of lateral shoots with 

 phyllotaxis in two rows on primary shoots with an arrangement in several rows. 

 Under this heading falls also the bilateral branching of leaves, whether those of 

 the stem itself be bilateral or multilateral. The dimensions of the punchun vegetationis 

 and the thickness of the axial structure derived from it may also be proportionate 

 to the number of the rows of lateral structures ; thus thicker mother-roots usually 

 produce three or more rows of secondary roots, while thinner primary roots produce 

 only two rows or even a smaller number. Thus, for instance, the roots of Crypto- 

 gams (according to NageH and Leitgeb), the thick primary roots of Zea, Phaseolus, 

 Pisum, Quercus, &c., form three, four, five, six, or more orthostichies of lateral roots, 

 which, on their part, are much thinner and produce fewer orthosdchies. The same 

 is not unfrequently the case with the phyllotaxis of stems. When the size of the 

 pimctum regetationis increases, the leaves are arranged in a larger number of 

 rows, as in the vigorous seedlings of many Dicotyledons, in Palms, Aspidium Filix- 

 7nas, &c. This is most strikingly exhibited in the many-rowed flower-heads of 

 the sunflower on the four-rowed foliage-stem ; the size of the punctum vegetaiioiiis 

 undergoing a sudden and great increase at the period w-hen the flower-head is 

 being formed (Fig. 108, p. 133). But, vice versa, the number of the rows of leaves 

 decreases when the size of the growing end of the stem decreases in consequence of 

 vigorous growth in length ; this is shown, for instance, in the few-row^ed long and 

 slender flower-stalks which proceed from the many-rayed leaf-rosettes of species of 

 Aloe, Echeveria, &c. If the insertion of the leaves or shoots embraces, at an early 

 stage, a large part of the periphery at the punctum vegetatio7ns, only a few rows of 

 leaves are formed ; if the insertion-planes are relatively small, the number of rows 

 on the axis increases. This is illustrated by the many rows of small flowers on 

 the spadix of Aroidese or in the racemes of TrifoHum, while the leaves of the same 

 plants are in few rows, their insertions embracing the stem or being even broader. 

 Hofmeister \ to whom we owe the introduction of this point of view in the theory 



^ Allgemeine Morphologic, § 11, where particular cases are discussed in detail. This treatise is 

 beyond question the most important that has hitherto been written on phyllotaxis ; nevertheless, in 

 my account, which necessary limits have confined almost to a mere sketchy I difter from Hofmeister's 

 views even in some points of primary importance. 



