ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 923 



for this inconsistency in the mode of treatment of axis and leaf. The 

 axis is, therefore, recognized by a distinct term, and the name phyllo- 

 podium is proposed for the main axis of the leaf, exclusive of its 

 branches ; the relation of the pinna to the phyllopodium is similar to 

 that of the leaf to the axis. In complicated leaves vre may distinguish 

 a liypopodium, a mesopodium which is the equivalent of the petiole, 

 and an epipodiiim, the equivalent of the upjjer part of the phyllo- 

 podium, exclusive of its branches. This method of study is shown 

 to be natural by the investigation of the development of leaves in 

 the lower vascular plants. 



In a series of types of Vascular Cryptogams and Gymnosperms it was 

 found that, in the simplest forms, the Hymenophyllaceas, the branching 

 is dichotomous ; in most Leptosporangiate Ferns the branching of 

 the leaf is at first monopodial ; the Osmundaceje are remarkable, and 

 probably unique, for having the two-sided apical cell replaced by one 

 which is three-sided and conical. In the Marattiace^ the phyllopodium 

 is from the first a solid structure. In the Cycadaceee the apex of the 

 phyllopodium is covered by a definite layer of dermatogen. 



There is, then, a progressive differentiation of the phyllopodium 

 as a supporting organ, and of other members of a higher order, which 

 develojDe as flattened organs. That of the former will perhaps throw 

 light on the mode of origin of the axis as a structiire bearing leaves. 

 " As the phyllopodium gi'adually asserts and, in the higher forms 

 of the above series, maintains its identity among the branches of the 

 leaf, so the axis may have differentiated itself as a supporting organ 

 from among members similar to itself in origin and development." 



Apical growth, which in the simpler forms is sometimes unlimited, 

 becomes restricted as we ascend the series; in the higher vascular 

 plants it ceases at an early stage, and there is a much gi'eater degree 

 of intercalary growth. It is concluded that the recognition of the 

 phyllopodium and treatment of the whole leaf as a simple branch or as 

 a branch system is in accordance with the true nature of the leaf as 

 seen in all vascular plants. The relation of the pinnae to the 

 phyllopodium is similar to that of the whole leaf to the axis which 

 bears it ; and this may be thus shown in a tabular form : — 



Shoot 



Axis Leaf 



I 



Phyllopodium Pinna 



Adcx of the Leaf in Osmunda and Todea.* — F. 0. Bower has 

 found in the young leaves of Todea superha and of Osmunda cinnamoniea 

 that the apex is occupied by a well-marked three-sided, conical, a2)ical 

 cell, from the three sides of which segments are cut off in regulai' 



♦ Proc Rov. Soc, xxxvi. (1884) pp. 442-3. 



3 p 2 



