CHAPTER XII 



GYMNOSPERMS 



120. Summary. The ferns, equisetums, and lyco- 

 pods are representatives of the third great division of the 

 plant kingdom, the Pteridophytes (fern plants). Their con- 

 tributions to the progress of plants are very important and 

 may be summarized as follows: 



(1) Leafy sporophytes. All Pteridophytes have leafy 

 sporophytes, and all Bryophytes have leafless ones, so that 

 this change is not only great, but also complete. The leafy 

 sporophyte means also a vascular system and roots, and 

 therefore these structures are introduced by the Pterido- 

 phytes. 



(2) Sporophylls.The setting apart of certain leaves to 

 bear sporangia makes a division of work between foliage 

 leaves and sporophylls, and the arrangement of the sporo- 

 phylis into the distinct cluster known as the strobilus marks 

 another advance. 



(3) Heterospory. The occasional appearance of hetero- 

 sporous plants among Pteridophytes, as Selaginella and a 

 few other forms, is noteworthy, because all the plants of the 

 next and highest group are heterosporous. Associated 

 with heterospory is a great reduction in the size of the two 

 gametophytes, which are so small that they project little if 

 at all from the spores which produce them. 



121. The four great plant groups. Three of the great 

 divisions of the plant kingdom have been considered. The 



207 



