THE LYCOPODS (CLUBMOSSES) 



mode of growth of the leaves. In British material the leaves grow spirally and are 

 pressed down to the ground. In the African specimens they are stiffly erect and con- 

 siderably longer. Whether this means that /. hystrix should be split into more species 

 than one is, however, for taxonomists to say. 



Fig. 2^6 b-d gives the cytological facts for the African and British material. Fig. 256c 

 shows microspore mother cells in /. hystrix from the Lizard, fixed in the field and subse- 

 quently sectioned and stained in haematoxylin. Fig. 2^6b shows a squash preparation 

 of the same material at the same magnification: n= 10. The diminutive chromosomes 

 and their low number make a very striking contrast with everything which we have 

 hitherto seen, and the comparison of chromosome size can equally be made from the 

 root of the Moroccan plant (Fig. 256^), in which approximately 20 somatic chromo- 

 somes are visible, at the standard magnification (1000 diameters), used almost through- 

 out this book. 



Before leaving Isoetes it may be of interest to add a note about the prothalli of/, hystrix, 

 since, as far as I am aware, these have never before been seen. They were detected 

 by Mr Ashby in the material from Morocco, and I am indebted to him for the 

 photograph in Fig. 257. A germinated megaspore from which the spore coat has become 

 detached is shown in Fig. 257 a, and the great length of the rhizoids is a striking 

 characteristic (see also Fig. 257^). Since rhizoids are absent from the aquatic species and 

 are as a rule poorly developed in the otherwise not dissimilar prothalli o^ Selaginella, their 

 presence here in association with the rather extreme habitat conditions of the species may 

 perhaps be a point of ecological importance. 



The second heterosporous genus, Selaginella 

 (Fig. 258), need not detain us long, since we 

 have only one species in Britain, S. spinulosa 

 A.Br., relatively common and vigorously 

 fertile in all our mountain districts. The only 

 cytological difficulty it presents is due to its 

 small size, but roots can be fixed at any 

 time and meiosis can be obtained in July. Fig. 258. Selaginella helvetica (L.) Link. 

 The chromosomes are, however, minute. Silhouette of a dried specimen of the plant 



,_^, . 1 /T-- \ 1 used, from Switzerland. Natural size. 



Iheir number (rig. 259a) appears to be 9 



at meiosis and 18 (Fig. 260c) in roots. These numbers were found again in the other 

 two European species. S. helvetica (L.) Link was fixed in the field in Switzerland in July 

 1938, and root tips gave the result shown in Figs. 259^ and 260^. S. denticulata (L.) Link 

 was collected in North Italy in the same year by Professor Lang, who brought it ahve to 

 Manchester, where it coned in cultivation. Fig. 259 c^ shows the side view of metaphase 

 in a megaspore mother cell in which the diminutive size of the group of chromosomes 

 is so extreme that they are scarcely visible. Their number, however, appears to be 9 

 in this cell and 18 as before in roots. 



This low chromosome number, comparing closely only with Isoetes hystrix among all the 

 living Pteridophyta so far studied, is a matter both of surprise and of importance. The 

 genus Selaginella is very large, with over 800 species, most of which are tropical. The three 

 European species, however, belong to widely different sections of the genus, S. spinulosa 



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259 



17-2 



