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- 
phylls into the distinct cluster known as the strobelus 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 
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