PTERIDOPHYTA. 715 



leaves above. Gradually the stem elongates and the adult form is assumed. The 

 special interest attaching to this stage is that it is characteristic of the mature 

 Phylloglossum referred to below. It has been suggested that the last-named genus 

 is a primitive form which retains as adult character what is but embryonic in 

 Lycopodium. 



The genus Phylloglossum (found in parts of Australia and New Zealand) possesses, 

 in addition to its tubercle and tuft of leaves, a stalk which terminates in a cone of 

 .sporangium-bearing leaves. There is only a single species. 



Psilotacece. — Includes two genera, Psilotum and Tmesipteris. Psilotum is tropical ; 

 it has delicate, angular, forking stems, and its leaves are reduced to tiny scales. It 

 is rootless and grows epiphytically. Its sporangia are three-chambered and are 

 borne on reduced leaves. Vegetative bulbils are frequently met with, especially on 

 those shoots which grow upon the substratum. Tmesipteris is also an epiphyte 

 (New Zealand and Australia). It has conspicuous, pointed leaves and long, trailing 

 stems. The ordinary leaves are simple, but the fertile ones fork like a V, and the 

 sporangium (which is two-chambered) is inserted on the upper surface at the junction 

 of the V. The prothallial stage is not known in either of these genera. 



Selaginellacece. — A family of some 300 to 400 species, which are in large part 

 tropical, and all belong to the genus Selaginella. The shoots are forked and are 

 dorsiventrally flattened. The leaves are borne in four rows — two rows of smaller 

 overlapping leaves right and left of the median dorsal line, and two rather larger 

 ones along the edges of the stem (c/. fig. Ill \ vol. i. p. 421). A very common 

 species in the alpine regions of Europe is Selaginella helvetica, whilst S. selaginoides 

 { = S. spinosa) is British. The last-named species, unlike the majority of Selaginellas, 

 is not flattened, and its leaves are distributed around the stem as in a Lycopodium,. 

 A characteristic feature is the presence of a little tongue inserted in the median line 

 of the upper surface of the leaf near its point of insertion; this is known as the 

 ligule. The roots in most cases arise, not directly from the stem, but from special 

 branches termed rhizophores. Selaginella is heterosporous. The sporangia are 

 spherical and arise in the axils of the fertile leaves, which are collected into cones. 

 The macrosporangia contain four macrospores, and the microsporangia numerous 

 microspores. Both kinds of sporangia occur usually in one cone, the former below; 

 or they may be in rows along the sides of the cones; or, finally, the two sorts of 

 sporangia may be on diflerent cones. 



The product of germination of a microspore consists of a single, simple anther- 

 idium, containing spermatozoids, which are provided with two flagella attached to 

 the pointed end. The macrospore produces a small, green female prothallium at 

 one end (as in the Hydropterideee, p. 710), whilst the rest of the spore, which here 

 divides into large cells, serves as a reserve of food-material. The green portion 

 bears the archegonia, and is exposed. After fertilization, an embryo arises, and 

 gradually develops into the >SeJagrmeZia-plant. The embryogeny presents various 

 features of interest. In particular may be mentioned the production of a suspensor 

 from that portion of the embryo which is towards the neck of the archegonium. 



