CYCADALES 



115 



The sorus arrangement is the typical fern arrangement. There are 

 no "synangia," like those of all the Marattiaceae except Angiopteris. 



The number of sporangia on a sporophyll is largest in the Cycas 

 region, and decreases to the Zamia end of the family. The numbers 

 of sporangia on a sporophyll, averaged from several counts are as 

 follows: Cycas media, 1,160; Dioon spinulosum, 770; Encephalartos 

 coffer, 567 ; Macrozamia miquelii, 503 ; Dioon 

 edule, 295; Microcycas calocoma, 245; Cera- 

 tozamia mexicana, 191; Stangeria paradoxa, 

 153 ; Bowenia serrulata, 67 ; and Zamia flori- 

 dana, 25. Sporophylls near the top and near 

 the bottom of the cone have fewer sporan- 

 gia, in Zamia, often only 2 or 3 on each side, 

 with a sterile area between. By hunting, 

 one can sometimes find a sporophyll with 

 only one microsporangium on each side, so 

 that it looks like a small megasporophyll 

 with its two ovules. The more reduced spo- 

 rophylls at the extreme top and bottom of 

 the cone are entirely sterile. 



In histological structure the microspo- 

 rangia bear a striking resemblance to those 

 oi Angiopteris (figs, iii, 112). In both, the 

 spores are very numerous, the stalk is massive, there are several layers 

 of wall cells between the epidermis and tapetum, dehiscence is similar, 

 and there is some ramentum. In the cycad, the ramentum is unicel- 

 lular, with rarely a cross wall; while in the fern, two or three cross 

 walls are common. The tapetum consists of very small cells in the 

 cycad and of rather large ones in the fern. The structure of the 

 sporangia, in both cases, is much like that of the eusporangiate ferns 

 and of the Cycadofilicales of the Carboniferous. In all the cycads the 

 microsporangium is strictly unilocular, in striking contrast with the 

 multilocular (synangium) type of the Bennettitales. 



The development of the sporangium is of the eusporangiate type. 

 There is a hypodermal archesporial cell or, in large sporangia, there 

 may be a row or plate; the archesporial cell divides, forming a pri- 

 mary wall cell and a primary sporogenous cell; from these, the thick 



Fig. 109. — Dioon edule: 

 side view and view of ab- 

 axial surface. The sori are 

 mostly in fours and threes; 

 X I.— After Chamberlain. '"^ 



