RELATIVE POSITIONS OF LATERAL MEMBERS, 1 79 



produce leaves, the latter roots. The horizontal position of the stem and its 

 bilateral development are here clearly the cause why the upper side 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 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; 

 but, 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 

 F/eris 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 cases very dif- 

 ferent. The effect of the difference of growth is first of all shown in the decidedly 

 horizontal position of the stem of the last-named plants, and also in the circum- 

 stance 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, but from certain products of their 

 division at a great distance from the apex of the stem, that the leaves originate. 

 This is common to Pteris and Salvinia ; but in the divisions of the segments 

 and in the total growth of the stem considerable differences between the two 

 occur. Pteris aquilhia forms on its thick underground horizontal shoots two 

 alternating rows of leaves standing almost on the upper side, 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 arrangement 

 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 

 punctum vegetati07iis 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 in Phanerogams, step by step, to the behaviour 

 of an apical cell. The most immediate visible causes lie rather in the position of 

 leaves already in existence, in their increase in breadth, in the form and size of the 

 vegetative cone, in its inclination to the vertical, and 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 enunciated by 

 him, that lateral shoots arise above the middle points of the largest gaps left by 

 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 mother-leaf, i. e, 

 the leaf in the axil of which they are produced. In Monocotyledons, for instance, 

 the first leaf of an axillary shoot usually stands on its posterior side, i. e. facing the 

 mother-axis ; in Dicotyledons, on the contrary, the axillary shoot generally begins 

 with two leaves, which stand right and left of the median plane of the mother-leaf, 

 and thus come in that free space between the mother-leaf and the primary axis 

 which is least exposed to pressure. 



As has now been shown by these brief indications, the investigations of 



N 2 



