THE STRUCTURE AND. DEVELOPMENT OF TRE 
SPOROPHYLLS AND SPORANGIA OF ISOETES. 
CONTRIBUTIONS FROM THE HULL BOTANICAL LABORATORY, 
XVII 
R. WILSON SMITH. 
(WITH PLATES XIII-XX) 
(Concluded from page 258) 
THE SUCCESSION OF SPOROPHYLLS. 
THE order of succession of the sporophylls is subject to some 
variation. It is not at all uncommon, especially in /. Engel- 
manni, to find the regular sequence interrupted by the occurrence 
of several megasporophylls among the microsporophylls. Occa- 
sionally, also, sporangia are found containing both megaspores 
and microspores. This is rarely the case in wild plants, though 
quite common, along with other irregularities, in those cultivated 
in the laboratory. Some plants taken in December, after grow- 
ing rapidly for seven or eight months in the laboratory, had 
formed only megasporangia; some others, though producing a 
few microsporophylls, had failed to bring any microspores to 
perfection. 
The sterile leaves of /. echinospora differ from the fertile ones 
chiefly in their smaller size, the reduction of the sheathing base, 
and the absence of a developed sporangium. They remain green 
throughout the winter; while the sporophylls, set free by decay 
of the base and buoyed up by the gas within the numerous air 
cavities, are borne away by currents or waves. A close study of 
the sterile leaves almost always reveals the presence of aborted 
Sporangia. These range in size from a few to many hundred 
cells; they are often of irregular shape and have lost their pro- 
toplasmic contents, though now and then one is found in which 
a few spores have matured. A longitudinal section of a typ- 
ical sterile leaf is shown in fig. 62, in which the shaded part 
— 323 
