200 MORPHOLOGY OF MEMBERS. 



only produces leaves ; and since its segments lie in two rows, there are two rows of 

 leaves, which we may imagine united by a zigzag line. But a further cause of the 

 difference, as compared with Fontinalis and Equtseium, 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 Fissidens, 

 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 a^id 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, /. 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 



