xxxvi] 



CYCADEOIDEA 



407 



synangia in rows. A transverse section just below the ovulate 

 portion of the cone shows a continuous ring of tissue encircling 

 the receptacle with vascular bundles parallel to the surface 

 (fig. 530, A 9 m); at a higher level the bundles fall into groups 

 preparatory to the breaking up of the disc into separate micro- 

 sporophylls. Immediately above the apex of the central cone 

 the coherent basal portion of the verticil of sporophylls is replaced 

 by the separate rachises which in section have the form of isosceles 

 triangles (fig. 530, B). Each microsporophyll is approximately 

 10 cm. long ; the longest pinnule being 1-5 cm. in length and bearing 

 two lateral series of synangia, 10 in each row. At maturity the 



FIG. 531. Cycadeoidea dacotensis. Synangia exposed by weathering and showing 

 the radially disposed loculi. (After Wieland; x 6.) 



microsporophylls probably spread out and the whole whorl was 

 thrown off leaving an annular rim (fig. 514, d) such as is often 

 seen at the base of older ovulate strobili. The immaturity of 

 the ovules in this (fig. 528) and other specimens led to the 

 suggestion that the strobili of Cycadeoidea were protandrous. 

 The synangia (fig. 531) are several times broader than long and 

 similar in size to those of the Fern Marattia. In each synangium 

 are two rows of elongated loculi (fig. 532) containing oval micro- 

 spores usually with a smooth exine and rather larger than those 

 of recent Cycads. The synangium-wall is composed of a palisade- 

 layer of thick- walled cells and a layer of smaller and thinner cells : 

 the palisade-layer is narrowest at the apex where dehiscence 

 occurs and broadest at the base where the hypodermal tissue is 

 more abundant and forms a short thick pedicel. The two rows 



