200 
MORPHOLOGY OF MEMBERS, 
only produces leaves ; and since its segments lie in two rows, there are two rows öf 
leaves, which we may imagine united by a zigzag line. But a further cause of the 
difference, as compared with Foniinalis and Equisetum^ arises from the fact that in 
Marsilea it is not every segment of the two rows on the upper side that forms a leaf; 
according to Hanstein, certain segments remain sterile, and these form the internodes 
which are at first wanting in Fontinalis and Equisetum, and are only formed at a 
later period by further differentiation and intercalary growth. In Pteris aquilina and 
in Salvinia the segments of the apical cell of the stem are also formed, as in Fisstdens, 
in two rows ; but the phyllotaxis is in all these cases very different. The effect of the 
difference of growth is first of all shown in the decidedly horizontal position of the 
stem of these plants, and also in the circumstance that the segments themselves grow 
vigorously in thickness and length, and divide before the formation of the leaves 
commences ; it is not from the segment-cells which are already in existence that the 
leaves originate, but from certain cells resulting from their division at a distance 
from the apex of the stem. This is common to Pteris and Salvinia; but in 
the divisions of the segments and in the whole growth of the stem considerable 
differences between the two occur. Pteris aquilina forms on the upper side of its 
thick underground horizontal shoots two alternating rows of leaves, while Salvinia 
forms alternating whorls on its slender floating shoots, the members of the whorls 
showing a very peculiar order of succession corresponding to the bilateral arrange- 
ment and the horizontal growth of the axis. 
The genetic forces which have an evident influence on the phyllotaxis of Cryp- / 
togams through the segmentation of the apical cell and the further behaviour of the 
segments, are wanting in Phanerogams, where the leaves spring from a small-celled 
cone of growth the tissue of which behaves like an almost homogeneous plastic mass. 
The immediate causes which determine the spot where a leaf or shoot is to arise 
can no longer be referred here, step by step, to the behaviour of an apical cell ; 
they lie rather in the position of leaves already in existence, in their increase in 
breadth, in the form and size of the cone of growth, in its inclination to the vertical, 
in its relation to the size of the mother-shoot, &c. — conditions which, as has already 
been mentioned under paragraph 5, have been treated in detail by Hofmeister. The 
rule there enunciated, that lateral shoots arise above the centres of the widest 
intervals between the youngest contiguous shoots, gives an efficient cause for the 
determination of the place of origin of new members, and may be applied also to the 
first leaves of lateral shoots, which generally show a definite relationship to the 
subtending leaf. In Monocotyledons, for instance, the first leaf of an axillary shoot 
usually stands on its posterior side, i. e. next the mother-axis ; while in Dicoty- 
ledons the axillary shoot generally begins with two leaves, which stand right and 
left of the median plane of the subtending leaf, and thus fall in the space between 
it and the primary axis which is least exposed to pressure. 
As has now been shown in this brief introduction, the investigations of 
phyllotaxis cannot at present do more than ascertain in each separate case the 
phenomena preceding and accompanying the origin of a member, as well as those 
forces which, from their direction, exercise an influence on the point of origin, 
and then lay down more general laws as the result of comparison in a sufficient 
number of cases. In these as in all other investigations into, organisms, we are 
