788 



SCIENCE 



[N. S. Vol. XLII. No. 1092 



shoots, there is a eonstruetionally dorsi- 

 ventral type of fern-rhizome. The leaves 

 of this alternate as in the leafy liverworts, 

 while the radial type of fern corresponds 

 to the moss-shoot. It is significant that 

 the early leaves of radially constructed 

 ferns usually exhibit a divergence of J in 

 the seedling, passing higher up the stem 

 into more complicated arrangements, and 

 the same is the case in mosses. I must not 

 enter into questions of phyllotaxy, but 

 may remark on the hopefulness of attack- 

 ing it from the study of the simpler shoots 

 of algae and Bryophyta rather than, as has 

 usually been done, beginning with the 

 flowering plants. 



In some ferns (the striking example 

 being Ceratopteris) the relation between 

 the segmentation of the apical cell and 

 leaf-production is as definite as in the 

 moss, each segment giving rise to a leaf. 

 This may hold more widely for ferns than 

 is at present demonstrated, and the ques- 

 tion deserves thorough reinvestigation to 

 ascertain the facts independently of any 

 theoretical views. That the coincidence 

 of the segmentation of the shoot expressed 

 by the leaf-arrangement and the segmen- 

 tation of an apical cell is not a necessary 

 one is, however, clearly shown in other 

 ferns, and is obvious in the case of shoots 

 with a small-celled meristem. The two 

 segmentations appear to be determined by 

 some deeper system of relations, which 

 may also be manifested in a ccenocytic 

 plant-body. 



In the complication of the uniaxial shoot 

 introduced by branching also there seems 

 to be an advantage in a wide area of com- 

 parison. The question most often dis- 

 cussed concerns dichotomous and mono- 

 podial branching. If the details of devel- 

 opment are to be taken into consideration, 

 the term "dichotomy" has usually been 

 very loosely applied. Apparent dicho- 



tomy, the continuation of one shoot by two 

 equally strong ones, is fairly common. But 

 in most cases investigated in detail the 

 branching seems to be really monopodial 

 and the forking due to the equally strong 

 development of a lateral branch close to 

 the main apex, not to the division of the 

 latter. In plants growing by a single 

 initial cell almost the only case of strict 

 dichotomy known is the classic one of Dic- 

 tyota. The branching of the ferns has 

 been the subject of numerous investiga- 

 tions, but there is a great lack of develop- 

 mental data. Usually the branches stand 

 in some definite relation to the leaves of 

 the shoot, behind, to one side, or on the 

 leaf-base, itself, the most interesting but 

 least common ease being when the branch 

 is in an axillary position. When the ma- 

 ture shoot only is considered, it is possible 

 to argue for the derivation of monopodial 

 branching from dichotomy or the converse. 

 Even the facts obtainable from the mature 

 plant, however, point to the dichotomous 

 branching being a modification of the 

 monopodial, the opposite view appearing 

 to land us in difficulties regarding the 

 morphology of the main shoot. It is un- 

 likely that a dichotomy involving the divi- 

 sion of the apical cell occurs in the fern- 

 shoot, and comparison with the Bryophyta 

 confirms the suspicion that the cases of 

 dichotomy are only apparent. 



In considering the construction of the 

 shoot we are at present limited to com- 

 parison of the normal structure and de- 

 velopment. The system of relations in the 

 shoot of the fern, affecting in the first 

 place the distribution of the leaves and 

 secondly that of the branches, appears, 

 however, to be of the same nature as in the 

 independently evolved shoots of Bryophyta 

 and algae. A morphological analysis based 

 on the simpler examples may lead on to 

 the experimental investigation of the com- 



