504 



A TEXTBOOK OF BOTANY 



[Ch. XII 



epiphytic. They branch freely, not from axillary buds but 

 by fission of the terminal bud, i.e. by dichotomy. The 

 tissues are well differentiated, but the fibro-vascular system 

 is primitive in type, for it consists really of one large central 

 bundle in which the xylem is completely surrounded by 

 phloem in the young stage, though they become intermingled 

 when older (Fig. 357). From such a bundle, by fragmenta- 

 tion, so to speak, has probably arisen the system prevailing in 



the higher plants, which 

 possess a ring of bundles, 

 each with phloem outside 

 of the xylem. The simple 

 sporangia, producing only 

 one kind of spores, occur 

 singly on the leaves, just 

 above the base on the 

 upper side; and accord- 

 ingly such leaves are typi- 

 cal sporophylls. There are 

 species in which every leaf 

 of the plant is a sporophyll, 

 and such cases are of in- 

 terest as showing a possible 

 mode of transition from a 

 sporogonium like that of Anthoceros (page 483), to the 

 sporophyte of the higher plants ; for one has only to imagine 

 that the green tissue around the spore chambers in Antho- 

 ceros (Fig. 343), grows out into leaves, carrying the separate 

 spore chambers part way with them, to have a condition like 

 that in Lycopodium. In other species, the lower leaves are 

 sterile, becoming typical foliage leaves, while the sporophylls 

 become collected together into clusters, called strobili, 

 sometimes with no abrupt transition to the leaves, but again 

 in definite, compact, stalked, and often club-shaped, clus- 

 ters. This is the highest condition these plants attain. The 

 sporangia open by slits and shed the spores which are dis- 



Fig. 357. — The stem of Lycopodium 

 complanatum, in cross section ; X 75. 



The stele, or central cylinder, contains 

 several masses of xylem, with inter- 

 mediate phloem ; these are branches from 

 one mass of xylem surrounded by phloem 

 in the young stem. (From Sachs.) 



