146 SPORANGIOPHORES AND SPOROPHYLLS 



the same median plane, and, excepting rare abnormalities, each sporophyll 

 subtends only one sporangium: this is seen in all plants belonging to 

 the Lycopodiales. But they illustrate various degrees of remoteness of 

 the sporangium from the axis, while still retaining the strict numerical 

 and subtending relation. Thus Selaginella shows the closest relation of 

 the sporangium to the axis ; but the sporangium of Lycopodium originates 

 clearly from the tissue of the sporophyll itself (Fig. 75): in some of them 

 (Z. Selago) the sporangium may at maturity appear to be thin-stalked 

 and axillary, while in others (Z. davatum, Lepidostrobus, and Isoetes) the 

 sporangium may extend with a broader base some considerable distance 

 along the upper surface of the sporophyll. An extreme condition is that 



FIG. 74. 



Selaginella Martetisii, Spring. Radial 

 section of a strobilus, including apex (ap), 

 and traversing a young sporophyll (/), and 

 sporangium (x). Also another section of 

 sporophyll and sporangium, rather older. 



FIG. 75- 



Radial sections through young sporangia of Lycopodium 

 Selago. In the youngest the whole sporophyll is shown (I), 

 and the axis (st), and it is seen that the sporangium arises 

 upon the surface of the sporophyll. X 200. 



of the early fossil Spencerites (Fig. 76), in which the narrow-stalkec 

 sporangium is attached some distance from the base of the sporophyll. Il 

 is thus seen that while the numerical and radial relations of sporangiui 

 and sporophyll are constant, the distance of the sporangium from the axi 

 may vary. This arrangement in the Lycopods, which dates back to th< 

 earliest fossil records, is certainly the simplest seen in the cones of 

 Pteridophytes, and the relation of the sporangium to the axis is habitually 

 closer in them than in any other type. 



But other plants, which also have representatives of palaeozoic age, 

 bear cones of more complex construction : these present intricate morpho- 

 logical problems if the effort is made to classify their parts according to 

 the strict categories and the usual successions of axis, leaf, emergence, 

 and sporangium ; for instance, the modern Psilotaceae and the ancient 



