XI V] ISOETES 61 



pit, but in I. hystrix this protective membrane is separated 

 from the base of the leaf by a narrow opening, the resemblance 

 of which to the micropyle of an ovule suggested to one of the 

 older botanists the employment of the same term*. Mr T. G, 

 HilP has called attention to the presence of mucilage canals in 

 the base of the sporophylls of /. hystrix, which he compares with 

 the strands of tissue known as the parichnos accompanying the 

 leaf-traces of Lepidodendron and Sigillaria in the outer cortex 

 of the stem. The transverse section shown in fig. 133, H and I, 

 shows two of these mucilage canals in an early stage of 

 development; a strand of parenchymatous elements dis- 

 tinguished by their partially disorganised condition and more 

 deeply stained membranes (fig. 133, I) runs through the 

 spandrels of the sporophyll tissUe close to the upper surface. 

 There is a close resemblance between the structure of these 

 partially formed mucilage-canals and the tissue which has been 

 called the secretory zone in Lepidodendron stems. Fig. 133, H, 

 also shows a large microsporangium with prominent trabeculae 

 (t) lying below the velum. A longitudinal section (fig. 133, E) 

 through a sporophyll-base presents an appearance comparable 

 with that of an Araucarian cone-scale with its integumented 

 ovule and micropyle. The megaspores are characterised by 

 ridges, spines, and other surface-ornamentation'. Though usually 

 unbranched, the perennial stem of Isoetes (fig. 132) has in rare 

 cases been found to exhibit dichotomous branching, a feature, as 

 Solms-Laubach* points out, consistent with a Lycopodiaceous 

 affinity. The apex is situated at the base of a funnel-shaped 

 depression. The stem is always grooved ; in some species two 

 and in others three deep furrows extend fi-om the base up the sides 

 of the short and thick axis towards the leaves: from the sides of 

 these furrows numerous slender roots are given off in acropetal 

 succession. A stele of peculiar structure occupies the centre of 

 the stem ; cylindrical in the upper part (fig. 133, A), it assumes 

 a narrow elliptical or, in species in which there are three furrows, 

 a triangular form in the lower portion of the tuberous stem. 

 The stem of /. lacustris represented in fig. 132, A, from which 



1 Braun (63). ^ Hill, T. G. (04) (06). 



3 For figures, see Motelay and VendrySs (82) ; Bennie and Kidston (88) 

 PI. yi. * Solms-Laubach (02). 



