5 20 BOTANY OF THE LIVING PLANT 



II. III.). The cotyledons already bear ligules (/, Fig. 413). The 

 further development is merely a matter of continued growth and 

 branching, which result in the establishment of a young Selaginella- 

 plant similar to the parent. 



If the embryology of Selaginella is compared with that of a Fern, 

 the most marked difference is seen to lie in the presence of the fila- 

 mentous suspensor. There are, however, certain primitive Ferns in 

 which a suspensor is present, as it generally is in Lycopods and Seed- 

 Plants. It seems probable that it is really a primitive organ, but that 

 it has been eliminated in the more modern Ferns, and some other 

 Pteridophytes, though retained by Seed-bearing Plants. 



The leading events in the life-cycle of Selaginella correspond clearly 

 to those of a homosporous Fern (see Diagram, Fig. 400). The micro- 

 phyllous sporophyte is the correlative of the megaphyllous Fern. 

 Sporangia are borne by both, and chromosome-reduction in the 

 formation of the spores establishes in both their haploid state. The 

 germinating spores produce the prothalli, and ultimately antheridia 

 and archegonia. After fertilisation a new sporophyte arises in both 

 cases from the zygote. All these events follow in the same succession 

 in Selaginella and in Dryopteris, and the several stages may be held 

 as homologous. The chief difference lies in the two distinct types of 

 spore in Selaginella. The distinction of these appears late in the 

 individual development. At first all the sporangia are alike, with 

 numerous spore-mother-cells in each. The microsporangia behave 

 like those of homosporous types, such as Lycopodium or Dryopteris, 

 all the spore-mother-cells undergoing tetrad-division and forming 

 microspores. But in the megasporangium of Selaginella only one of 

 them as a rule shows tetrad-division, the rest giving up their substance 

 to that one (Fig. 410). The result is that each individual spore of 

 the megasporangium is hypertrophied as a megaspore, and stored 

 with nutritive material. The conclusion to be drawn from these 

 facts is that the difference between these spores, which are then 

 described as heterosporous, has been secondarily acquired from the 

 primitive homosporous state, and that it is founded on nutrition. 

 Comparison supports this inference : for there is evidence of like 

 progressions from the homosporous to the heterosporous state in the 

 Equisetales and in the Filicales. In the latter, the Hydropterideae 

 also show identity of early development of the sporangia, whether 

 they are to be megasporangia or microsporangia. It is concluded, 

 therefore, that heterospory has been homoplastic in distinct phyla : 

 that is, that it has been initiated more than once in Descent. 



