PRINCIPAL GROUPS OF PLANTS. 



77 



The Life History of this group of plants may probably be 

 best illustrated by following that of a moss plant. Beginning 

 with the germination of an asexual spore which is microscopic in 

 size and which germinates on damp earth, there is produced an 



FIG. 45. A common moss (Polytrichum gracile). A, showing leafy branches (gameto- 

 phores) two of which bear sporogonia, a detached sporogonium (sporophyte) with sporan- 

 gium from which the calyptra (ca) has been detached. B, longitudinal section through a 

 nearly ripe sporangium showing columella (o), the elongated area of sporogenous tissue 

 (archesporium) on either side, annulus (n), peristome (p), lid or operculum (u); C, trans- 

 verse section of sporangium showing columella in center and dark layer of sporogenous 

 tissue (archesporium); D, ripe sporangium (capsule) showing the escape of spores after 

 detachment of lid; E, ripe spore containing large oil globules; F, ruptured spore showing 

 separated protoplasm and oil globules; G, two germinating spores 14 days after being sown, 

 showing beginning of protonema in which are a number of ellipsoidal chloroplasts. After 

 Dodel-Port. 



