684 PROFESSOR F. O. BOWER ON 



same is the case for the older leaves, and it is illustrated in the successively larger 

 pinnse which they bear. Thus in fig. 18, b, the apical region of two older leaves is 

 shown, with their distal scorpioid sympodium. The branches entering the upper- 

 most pinnae show a single forking ; but passing downwards clear steps are seen 

 leading to a scorpioid sympodium in each, with a marked midrib in the lowest, and 

 even a secondary lobation. Thus is initiated a leaf-construction which may be 

 pursued ad infinitum. Though relatively simple, it is to be interpreted on the same 

 lines as in the previous cases. 



It may be noted that in the earliest pinnse there is some inconstancy in the 

 venation, which is sometimes anadromic (pinnse marked ( X ) in fig. 18,6, c), but much 

 more frequently it is katadromic, and the ' latter seems to be the constant condition 

 to which it settles down in the mature plant : it is clearly indicated in the lower 

 pinnse of fig. 18, b. These simple examples of instability, in a character which has 

 often been used as distinctive, throw light on what is the real nature of it. Where 

 the vein of a pinna has forked, either of the two shanks may take the further lead. 

 Such simply forked pinnae are seen in all the plants here represented. If the acro- 

 scopic shank takes the lead and forks again, the other appears as an appendage, and 

 the venation would be termed " katadromic." If the basiscopic shank takes the lead 

 and forks again, the acroscopic shank appears as an appendage, and the venation is 

 " anadromic." Such instability as Gleichenia shows may be considered intelligible 

 in a Fern which in its early stages has not advanced far from the primitive equal 

 dichotomy. 



The young plants of G. fulva afford an insight into the origin of the well-known 

 " false dichotomy " of the leaves in the genus. Three young plants have been drawn 

 (fig. 18, c, d, e), and in each of them the foot is still recognisable. The leaf 

 opposite to it is the cotyledon (cot.), and in fig. 18, e, the insertion of its vascular 

 strand proves that it is so. In (c) the cotyledon is small and its apex fully 

 developed, as is also that of the second leaf; but in the third, though it has 

 developed four pinnse, it has its apex still curled up, and, as no pinnse of reasonable 

 size are seen upon it, it may be held to be in a state of arrest. In fig. 18, d, a plant 

 is shown in which the second leaf is in a corresponding state. In fig. 18, e, it is the 

 cotyledon itself that is arrested, while the next leaf has its apex fully formed. This 

 cotyledon is shown on a larger scale in fig. 18, e, and it appears already to illustrate 

 the fundamental features of the Gleicheniaceous leaf. Right and left are pinnse 

 fully formed, though on a quite rudimentary plan, while between them is the 

 arrested tip, representing the whole upper region of the sympodium. Dichotomy 

 is in actual fact present, and it is shown with equal shanks in each of the 

 lobes. Comparison with other Ferns shows that dichotomy underlies the whole 

 sympodium of this leaf. But the apparent or "false dichotomy" of the Gleichenia 

 leaf is not a simple dichotomy at all ; it is a result of arrested development in a leaf 

 constructed throughout as a scorpioid sympodium. These observations accord with 



