GROUPS OF PLANTS. 



47 



(Fig. 28, A). The leaf-bearing axis varies considerably in size, 

 in some cases it is but a millimeter high whereas in some species,' 

 as Polytrichum (Fig. 28), it may be several hundred millimeters 



Fig. 28. 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, 

 transverse 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. 



