8o 



MORPHOLOGY OF GYMNOSPERMS 



The general and apparently inevitable interpretation has become 

 more assured since the discovery of Cycadofilicales. In that group many 



of the seeds are borne terminally 

 upon reduced filiform branches 

 of a modified frond; and it is 

 natural to infer that the seed pedicel 

 of Bennettitales represents such a 

 frond reduced to its lowest terms, 

 that is to a single seed-bearing stalk. 

 This means that the seed pedicel 

 is a foliar rather than an axial struc- 

 ture, and that it is a case of exces- 

 sive reduction, which has resulted 

 in a simple, one-seeded megasporo- 

 phyll (carpel). There seems to be 

 no better way of disposing of the 

 interseminal scales than to regard 

 them as sterile and specialized 

 megasporophylls. One of the most 

 striking features of Bennettitales, 

 as contrasted with Cycadofilicales, 

 is the relatively slight reduction 

 of the microsporophylls, associated 

 with the very great reduction of 

 the megasporophylls. 



The structure of the seed is what 

 has come to be spoken of as the 

 cycadean type (figs. 67-69). There 

 is a distinctly three-layered testa 

 (outer and inner fleshy layers and 

 a middle stony layer), which is 

 evident chiefly in the region of the 

 integument free from the nucellus 

 (29). The inner layer of the 

 testa is said to form the lining 

 of the long micropylar tube, which 

 reaches to the surface of the ovulate 



Fig. 64. — Cycadeoidea Wielandii: 

 radial longitudinal section of seed-bearing 

 strobilus, witH partially restored bract 

 tips; the arrow indicates direction vertical 

 to the trunk, the section passing through 

 the exact median and vertical plane of 

 the axis of fructification; m, x, p, and c, 

 respectively the medulla, xylem, phloem, 

 and cortex of the trunk as seen in radial 

 longitudinal section; a, insertion of armor 

 on cortex; I, old leaf bases; d, insertion 

 of dehiscent hypogynous disk; s, erect 

 seed; 6, hair -covered bract.— 'Figure and 

 description after Wieland (22). 



