Chapter VIII 



EMBRYOGENESIS IN THE LYCOPODINEAE : 

 SELAGINELLA AND ISOETES 



SELAGINELLA 



Q1ELAGINELLA is a large, coherent and widely distributed genus of 

 O ancient origin, having evident affinity with the fossil Selaginellites. 

 The species, which number upwards of 600, show a considerable range 

 in size, symmetry and morphology, e.g. from the small, radially sym- 

 metrical, temperate S. spimdosa, to the large, climbing, dorsiventral, 

 tropical S. Willdenovii. Thus far there is little information on the inter- 

 relationships of the species as revealed by genetical or cytological 

 studies. Manton (1950) has shown that the three European species, 

 S. spinulosa, S. helvetica and S. denticidata, have low chromosome 

 numbers, with /7=9. In Isoetes hystrix, /7=10. According to Manton: 

 The pteridophytes seem almost certainly to have possessed in the past, 

 and to have retained in these two genera, the simple nuclear state that 

 the modern Flowering Plants still for the most part display.' These low 

 numbers in plants of ancient stock are perhaps surprising, the more so 

 in that these genera show what is generally accepted as the considerable 

 advance to the heterosporous condition. 



THE EMBRYONIC ENVIRONMENT 



In Selaginella the embryo is nourished by the prothallus or gameto- 

 phyte formed within the large megaspore while the latter is still inside 

 the sporangium. In this respect, Selaginella resembles some of the 

 primitive seed plants. There are indications that carbohydrates 

 accumulate considerably more rapidly than proteins in the megaspore, 

 and this may affect the subsequent embryonic development. In the 

 young megaspore of S. helvetica, according to Fitting (1900), the 

 megaspore walls outgrow the cytoplasm and the cytoplasmic film 

 becomes rounded off" as a spherical vesicle attached to the endospore 

 wall. On further development this vesicle begins to increase in size, 

 many free nuclear divisions take place, and the cytoplasm again makes 

 contact with the endospore membrane. Many of the nuclei now move 

 towards the apical end of the megaspore and active growth of the 

 cytoplasmic layer ensues. Thereafter the synthesis of protoplasm takes 

 place rapidly and the central vacuole of the spore disappears. Cell 

 formation, i.e. the formation of gametophyte tissue, begins at the 



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