42 AN INTRODUCTION TO THE STUDY OF FOSSILS 



10. How do plants secrete lime ? 



11. Name the important lime-secreting groups of Algae with 

 two genera under each. 



12. How important are Algae as rock-builders? 



13. What are lichens ? Their geologic age ? 



DIVISION II, BRYOPHYTA 



The Bryophyta exhibit a distinct advance upon the Thallo- 

 phyta in the greater soecialization of the plant body and in the 

 different method of reproduction. As the algae are essentially 

 water-dwelling plants there is little necessity for specialization 

 of organs since any of the plant's cells can serve for absorbing 

 the food which surrounds it on all sides. With the evolution 

 of the bryophytes there came a change from aquatic to terres- 

 trial conditions and there thus arose the need for specialized 

 organs adapted to getting the food from the soil and from the 

 air. The adaptation to terrestrial conditions being still imper- 

 fect, however, many of the bryophytes, such as some of the liver- 

 worts, still have thalloid plant bodies, w^ithout distinction of 

 root, stem and leaves, wMe the group as a whole is moisture- 

 loving (Fig. 11). 



The most distinct advance, however, of the bryophytes upon 

 the thallophytes is in the method of reproduction, — in the estab- 

 lishment of a distinct alternation of generations. This method 

 is illustrated by the life history of some common moss, such as 

 the hair-cap {Polytrichum commune, Fig. 11, C). This plant, 

 as we commonly see it, consists of a green stalk bearing many 

 tiny leaves, and, if the plant is fruiting, it supports at its apex a 

 slender bristle-like seta which ends in a sac (capsule) containing 

 spores. When the spores fall on moist earth they may ger- 

 minate by sending out a mass of green threads, the protonema, 

 and from this grow the tiny moss plants. These plants bear the 

 male and female reproductive organs and from the union of their 

 products, — the fertilized egg cell, arises the spore-bearing part 

 of the plant, i.e. the capsule borne at the summit of the seta. 



Thus there is an alternation of the asexual or sporophyte 



