V] 



MONOPODIAL BRANCHING 



91 



(ii) sympodial dichotomy ; (iii) monopodial branching. This succession 

 probably reflects truly the steps in the phyletic elaboration of the foliar 

 structure in the Filicales at large. A comparison of this monopodial 

 branching of the leaf with the 

 monopodial branching of the axis 

 described in Chapter IV shows that 

 both cases are alike in being refer- 

 able in origin to dichotomy, and 

 follow on its sympodial develop- 

 ment. Accordingly, the general 

 conclusion for the branching of the 

 leaf is, that monopodial branching, 

 wEich is characteristic of Angio- 

 spermic leaves, and is seen in the 

 lozver a nd first formed pinnae of 

 fnofT}/ Ferns, arrived last in the 

 course of evolution, arising as a 

 nwdijication of dichotomy, whicJi 

 still survives dis tally in most leaves 

 of Ferns. 



In some leaves of Ferns, how- 

 ever, the final transition to dicho- 

 tomy at the apex does not take 

 place at all. The apical growth 

 appears to be arrested before the 

 transition to the more primitive 

 type of branching, but after the 

 monopodial origin of the lower pinnae has been carried out. The phyllo- 

 podium then appears to end in a blunt cone, as may be seen in the leaves 

 of Angiopteris (Fig. 86), (71). This is precisely what happens in the 

 developing leaves of Cycads, and in many pinnate Angiosperms, in 

 which the phyllopodium does not explain its sympodial origin by any 

 transition to apical dichotomy, as it does in so many Ferns. This arrest 

 of the apical growth of the leaf in certain Ferns, and its habitual adop- 

 tion in the Cycads and in the Angiosperms, throw into all the greater pro- 

 minence the continuance of that growth which is habitual in the Ferns 

 themselves: and especially those cases where it is unlimited, as in Gleichenia 

 and Lygodium. No other series of Vascular Plants, excepting the Pterido- 

 sperms, has shown continued apical activity of its leaves in like degree. This 

 marks them off sharply from the Lycopodiales, Equisetales, and Spheno- 

 phyllales: in fact it supplies the most distinctive feature of their vegetative 

 system among Vascular Plants. 



Fig. 85. Allosorus crispus. Outline of a leaflet. The 

 branching is clearly dichotomous. The apex has 

 divided into lobes, i and 2, of which i is the stronger 

 and continues the growth ; 2 forms a lateral lobe. 

 Below we have lobes 3 and 4 which have been simi- 

 larly formed. The leaf-spindle (phyllopodium), .S", 

 is only a narrower portion of the lamina which is 

 mechanically strengthened later. Magnified. (After 

 von Goebel.) 



