RELATIVE POSITIONS OF LATERAL MEMBERS. 
197 
members is preceded by circumstances connected with their development which assist 
in determining their place of origin. Of this nature, for instance, is the connexion 
between the points of origin of lateral roots and the fibro-vascular bundles, the 
course of which determines the arrangement of the roots in rows ; and this in 
turn determines the lateral roots being arranged spirally or in whorls. Here 
the arrangement in longitudinal rows is clearly the general and primary one; the 
divergences and longitudinal distances are a secondary effect 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, since it must 
be formed beneath, beside, or above its median plane; forces of secondary import- 
ance then 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 a distichous 
phyllotaxis on primary shoots with an arrangement in several rows. Under this 
heading falls also the bilateral branching of leaves, whether the stem itself be bilateral 
or multilateral. The dimensions of the growing point and the thickness of the axial 
structure derived from it may also determine the number of the rows of lateral 
structures; thus thick mother- roots usually produce three or more rows of secondary 
roots, while more slender primary roots produce only two rows or at all events a 
smaller number. Thus, for instance, the roots of Cryptogams (according to Nageli 
and Leitgeb), the thick primary roots of the maize, oak, pea, scarlet-runner, &c., 
form three, four, five, six, or more orthostichies of lateral roots, which, on their part, 
are much slenderer and produce fewer orthostichies. The same is not unfrequently 
the case with the phyllotaxis of stems. When the size of the grov/ing point increases, 
the leaves are arranged in a larger number of rows, as in the vigorous seedlings of 
many Dicotyledons, in Palms, Nephrodium Filix-mas, &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 growing point undergoing a sudden and great increase at the 
period when the flower-head is being formed (Fig. 126, p. 171). But, vice vers'd, 
the number of the rows of leaves diminishes when the size of the growing end of the 
stem decreases in consequence of vigorous growth in length- this is seen, for 
instance, in the few-rowed long and slender peduncles which proceed from^ the 
many-rayed leaf-rosettes of species of Alo'e\ Echeveria, &c. If the insertion of the 
leaves or shoots takes up, at an early stage, a large part of the periphery at the 
growing point, 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 in the spadices' of Aroideae or the racemes of Trifolium, 
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 of phyllotaxis, states the general rule in the following 
^ Allgemeine Morphologie, § 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 sketch, I differ from Hofmeister's 
views even in some points of primary importance. 
