518 



BOTANY 



PART II 



traced back to the Triassic period. Developed partly as land, partly as swamp 

 plants, they may always be distinguished by the characteristic structure and 

 habit of the asexual generation. They have a branching, underground rhizome 

 on which arise erect, aerial haulms, usually of annual growth. The rhizome of 

 the common Horse-tail, Equisetum arvcnse, develops also short tuber -like 

 branches which serve as reservoirs of reserve material and hibernating organs 

 (Fig. 486). The aerial haulms remain either simple, or they give rise to branch 

 whorls, and these in turn to whorls of a higher order. All the axes are 

 formed of elongated internodes ; they have a central pith-cavity and a peripheral 



series of smaller air channels. The col- 

 lateral vascular bundles form a single 

 circle, as seen in transverse section (Fig. 

 485). 



At each node is borne a whorl of scale- 

 leaves pointed at the tips, and united 

 below into a sheath closely enveloping 

 the base of the internode. The lateral 

 branches are developed in the axils of the 

 scale-leaves, but not having space to 

 grow upwards they pierce the narrow 

 sheath. As a result of the reduction of 

 the leaf laminae, the haulms themselves 

 assume the function of assimilation, and 

 for that purpose their cortical tissue under 

 the epidermis is provided with chlorophyll. 

 The SPOKANGIA are borne on specially - 

 Fio. 4S5.-Equisctum arvense. Transverse sec- shaped leaves or sporophylls. The sporo- 

 tion through the stem, m, Lysigenic medul- phylls are developed in whorls, but are 

 lary cavity ; c, endodermis ; d, carinal canals closely aggregated at the tips of the erect 

 in the collateral bundles; vl, vallecular f ert ile shoots into a cone (Fig. 486), which 

 cavities ; hp, sclerenchymatous strands m . 



the furrows and ridges ; ch, tissue of the ls sometimes spoken of as a flower, from 

 primary cortex containing chlorophyll ; st, the correspondence in its structure to 

 rows of stomata. (x 11. After STRAS- the male flower of the Conifers. The 

 BURGER.) lowest whorl is sterile, and forms a collar- 



like protuberance. The sporophylls (Fig. 



486 E, C) are stalked and have a peltate expansion, on the under side of which are 

 borne the (5-10) sac-like sporangia. In the young sporangium the sporogenous 

 tissue is surrounded by a wall consisting of several cell layers, but eventually the 

 tapetal cells of the inner layer become disorganised, and their protoplasm penetrates 

 between the developing spores, forming the periplasmcdium. At maturity the 

 wall of the sporangium consists only of the outermost of the original layers ; the 

 cells of this are provided with annular and spiral thickenings. The sporangia thus 

 resemble the homologous pollen-sacs of Phanerogams. The dehiscence is determined 

 by the cohesive force of the diminishing amount of water in the cells of the outer 

 layer and' the contraction of the thin parts of the cell walls on drying. The 

 sporangia split longitudinally, and set free a large number of green spores, which 

 are nearly spherical in shape, and have peculiarly constructed walls. In addition 

 to the endospore and exospore, the spores are overlaid with a perispore deposited 

 by the periplasmodium, and consisting of two spiral bands (elaters) which are 

 attached to the spores only at their point of intersection (Fig. 486 D). On drying, 

 the spiral bands loosen and become uncoiled ; when moistened they close again 



