﻿2 IO 



RELATIONSHIPS. 



in habit of frond emergence from the armor, some having nearly fully grown 

 pinnules while the petiole is very short {Dion, Macrozamia), others reaching consid- 

 erable petiolar length while the pinnules are still immature (Cycas, Zamia). The 

 presence of well-formed pinnules while the leaf is still within the armor is, as has 

 been seen, a well-marked character of the Cycadeoidece, to which, in the case of 

 silicified trunks, is fortunately due the not infrequent preservation of both isolated 

 and entire crowns of young fronds in very great perfection. This is identically 

 the condition in Cycadeoidea and Cycadella, and the erect position is also to be 

 seen in the fern Botrychium. Prefoliation is direct in Dion, Encephalartos, Cera- 

 tozamia, and Macrozamia, the rachis being straight and the pinnules folded back 

 to face, so that the successive blades imbricate from the base to the tip of the leaf 

 rankwise on each side of the midrib, the ranks facing next to the stem axis, and 

 the rachis being distal to it. In Cycas the rachis is straight, but the pinnules are 



Fig. 121. — Very young (ronds of existing Cycads, showing types oi pinnule succession. X 10. All from Bower. 



a. Cycas Seemanni. Horizontal section through apex of a plant 12 months old. Leaves 1 to 7 are present as numbered ; 

 a, apex of stem ; w, wing on basal sheath of frond. 



b. Cycas Seemanni. Young frond showing basal wings (w). and incipient pinnules in irregular succession with respect 



to size. 



c. Cycas Jenkinsoniana. Young frond in which the formation of pinnules has begun, those a little below the middle being 



the most advanced, and the succession hence a mixed one. or partly acropetal (from below up), and partly basipetal 

 (from above down). 



d. Macrozamia Miquelii. Developmental stage of young frond, showing an exclusively basipetal order of pinnule succession. 



circinately rolled, as in most ferns. Conversely, in perhaps all species of Zamia 

 and in Stangeria, whilst the pinnules are straight the rachis is once-inflexed (see 

 figure 47), as likewise in the megasporophylls of Cycas. This is the exact condi- 

 tion seen also in the microsporophylls of the stamiuate disk of the Cycadeoidea?. 

 Hence, while there is no case in which both axis and pinnules are circinate, verna- 

 tion may be considered as in general of a filicinean character. The uuexpanded 

 leaves as enumerated exhibit four conditions : 



(a) Rachis and pinnae (bearing pinnules) infiexed: Bowenia. 



(b) Rachis erect, pinnules circinate: Cycas. 



(c) Rachis inflexed (or sub-circinate), pinnules straight: Stangeria and Zamia. 



(d) Rachis erect, pinnules erect : Ceratozamia, Dion, Encephalartos, Macro- 



zamia. (Also Cycadeoidea and Cycadclla amongst the Cycadeoideae.) 

 The leaves or fronds, as organized into the palm-like crown borne at the sum- 

 mit of the cycadean trunk, may vary in number from a few to more than a hundred; 



