178 Practical Plant Biology. 



female gametophytes are indistinguishable, and it is possible that 

 external conditions decide the sex of the plant. In Selaginella the 

 external difference of the spores at the time of shedding shows that 

 the sex of the gametophyte is already decided in the spore, and 

 external conditions are powerless to alter it. Plants which are thus 

 characterised by two kinds of spores, each giving rise to sexually 

 distinct gametophytes are called heterosporous. They are found 

 among the higher fern-like plants ; and all the Seed Plants are 

 heterosporous. 



It has already been noticed that the spores of Selaginella begin 

 their germination in the sporangium. In one species not only the 

 germination but the whole development of the female gametophyte 

 takes place in the megasporangium. The microspores are shed 

 from the microsporangia and fall on the lower sporophylls of the 

 cone. When the cone is moistened with dew or rain the sperms 

 are liberated and make their way to the archegonia exposed by the 

 rupture of the megaspores still remaining in the megasporangium. 

 Fertilisation takes place there, and actually young sporophytes, 

 developed within the gametophyte, may be seen emerging between 

 the sporophylls of the cone. The retention of the megaspore in 

 the megasporangium until fertilisation of the gametophyte is effected 

 is a state of things characteristic of the Seed Plants. In this 

 respect the heterospory of Selaginella forms a connecting-link 

 between the Seed Plants and the Ferns. 



PRACTICAL WORK. 



Sketch a piece of Selaginella martensii showing the prostrate branching 

 system, the arrangement of the leaves, the rhizophores and roots. 



Make transverse sections of the stem, and record your observations on 

 them in sketches with the low and high power. 



Mount several leaves in water. Look for the ligule. Examine the under 

 surface of the leaf and note the stomata, and the presence of single chloro- 

 plasts in some of the cells. 



Pick off some of the cones and boil them in dilute potassium hydrate to 

 clear them. Mount them in water and examine. The microsporangia and 

 megasporangia will be seen, and the ligule on some of the sporophylls. 



Cut longitudinal sections of the cones and supplement with these your 

 observations with the cleared specimens. 



Shake out some of the spores on to a piece of filter paper in a Petri dish. 

 Moisten with rain-water and keep in a warm room for a few weeks. Examine 

 at intervals. Notice the bursting of the spores and the female gametophyte 

 protruding from the megaspore and later the stem and first two leaves and 

 the root of the embryo sporophyte. 



