242 PLANT LIFE 



Even among plants nearly related to the 

 ferns we find that the prothalli produced by 

 the spores tend to differ in their capacity for 

 growth. Those which are destined to produce 

 eggs are large and well stocked with food, those 

 which will produce the sperms are small. 

 This difference becomes reflected in the 

 spores, and even in the sporangia. 



In the Selaginella plants, often grown in 

 greenhouses, certain of the sporangia produce 

 a large number of small spores, whilst others 

 become much larger, but only bring a very 

 few (usually four) large spores to maturity. 



The small spores form a rudimentary 

 prothallus and a larger or smaller number of 

 sperms. It is essential that the number of 

 small spores should be kept up, so as to 

 maintain a fair chance of a sperm from one of 

 them reaching the relatively few available 

 female prothalli. 



Passing to the flowering plants, it is difficult 

 at first sight to realise that we are only 

 witnessing the final stages of an evolutionary 

 development of the structures so clearly 

 distinguishable in the fern. But so it is, and 

 it will be of interest to trace, even briefly, and 

 in spite of the wide gaps, the points of resem- 

 blance between them. 



The obvious starting-point in both cases is 

 the fertilised egg. The fern plant and' the 

 flowering plant each spring from this source. 

 The fern closes its life cycle by producing 

 spores in the way we have seen. The flowering 



