Lecture XXIV. 213 



The heterospory of Selaginella produces a slight differentiation 

 in the later stages of the sporangia. The four large megaspores 

 distort the shape of the containing sporangium, so that as the 

 spores come to maturity it is distinguishable from the smaller and 

 smoother microsporangium. The differentiation of the sporangia 

 is much more complete in the pine, and there is little similarity 

 in form between the microsporangia or pollen-sacs and the mega- 

 sporangia or ovules. Furthermore, the leaves with which the two 

 kinds of sporangia are associated are widely different in form and 

 are easily distinguished as carpels or megasporophylls and stamens 

 or microsporophylls. As in Selaginella the sporophylls are grouped 

 together on axes to form cones or strobili but, while in Selaginella 

 the microsporophylls and megasporophylls occur together in the 

 same strobilus, in the Scots Pine they are separate and form micro- 

 strobili (pollen-cones) and mega-strobili (seed-cones). In other 

 pines these two kinds of strobili occur on different trees or sporo- 

 phytes, and thus there is a differentiation into microsporophytes 

 and megasporophytes. 



Thus we may trace in the Archegoniates and Seed-Plants a 

 series beginning with the simple differentiation of the sexual cells 

 formed on the same gametophyte as is found in Aspidium, passing 

 to the differentiation of not only the gametes but also of the gameto- 

 phytes, and finally leading up to those cases where two kinds 

 of sporophytes are developed as well as two kinds of gameto- 

 phytes. 



The comparison of the individuals belonging to these different 

 groups, the tracing of their life-histories and. the establishment of 

 the various homologues exhibited by each has contributed much 

 to the philosophy of botany, and has controlled speculation by 

 exact knowledge. 



The occurrence of the alternation of generations in so many 

 different groups of plants, its modifications and. the extraordinary 

 gradations connecting these modifications with one another con- 

 stitute a problem which ever and anon obtrudes itself on the re- 

 flecting mind. Why should the life-histories of the different 

 groups of plants resemble each other in their broad outlines? 

 Why should intermediate forms be found connecting the most 

 diverging extremes ? What is the underlying principle responsible 

 for the existence of homologues ? 



Although we are still far from a complete answer to these 

 questions, the endeavour to find a solution for them has con- 

 tributed, as we shall see later, some of the most elevating specu- 

 lations of biological science. 



