224 



THE ANATOMY OF WOODY PLANTS 



the section of the megaspore membrane. In addition to the 

 germinated megaspore shown in the figure, three other abortive 

 megaspores make their appearance at an earlier stage, and these 

 have thicker walls than the functional spore. These are not shown 

 in the late stage of development appearing in the figure. Not only 

 does the megaspore retain its thick membrane in the fructification 

 of Lepidocarpon, but, contrary to the conditions found in typical 



seeds, the mechanical layer of 

 the megasporangium likewise is 

 well developed and was prob- 

 ably capable of dehiscence. 

 The integument with which 

 true seeds are provided is rep- 

 resented in this foreshadowing 

 of seminal structure by the 

 upfolded edges of the sporo- 

 phyll. The absence of an 

 apparatus for receiving the 

 microspores or pollen likewise 

 differentiates the fructification 

 under consideration from the 

 seeds of even the lowest of the 

 seed plants. 



In Fig. 1 66 is shown the 

 longitudinal view of another 

 seedlike structure from the Carboniferous of England known as 

 Miadesmia. Here the resemblance to the real seed is much more 

 marked than in Lepidocarpon. The sporophyll so completely 

 involves the megasporangium that only a small aperture is left 

 which corresponds physiologically, although not morphologically, 

 to the micropyle of the seed in the true seed plants. Within the 

 "integument" is inclosed, not only the megasporangium, but also 

 the ligule. The megasporangium is much less typical than that of 

 Lepidocarpon, for its mechanical layer fails to develop and it pro- 

 duces only one spore in contrast to the four that come into 

 existence in the case of the Lepidocarpon, There is no good reason, 

 however, to regard the structure here described as representing a 



FIG. 165. Seedlike sporangium of 

 Lepidocarpon (after Scott). 



