8KCT. I 



CRYPTOGAMS 



455 



sporangium remains attached to the spore in the form of an umbrella-like expansion. 

 The formation of the prothallia is effected in essentially the same way as in 

 Salvinia, except that only one autheridium with eight spermatozoids arises on 

 each of the small male prothallia protruding from a massula. 



The sporocarps of the Marsiliaceae have a more complicated structure : those of 

 Pilularia globulifera are divided into four chambers, each with a single sorus ; in 

 Marsilia they enclose numerous sori (14-18) disposed in two rows. The sori in 

 both genera contain both micro- and macrosporangia, while those of the Salvini- 

 aceae always consist of only one kind of sporangium. 



In the case of the Marsiliaceae the prothallia are even more reduced, but 

 otherwise their mode of development is very similar. Each of the minute female 

 prothallia formed at the apices of the macrospores produces a single archegonium. 



CLASS II 

 Equisetinae (Horse-tails) ( 116 ) 



The Equisetinae, which form an entirely independent class, include only the 

 one genus Equisetum, comprising 20 species, found widely distributed over the 

 whole world. Developed partly as land, partly as swamp plants, they may always 

 be distinguished by the characteristic 

 structure and habit of the asexual genera- 

 tion. They have a branching, underground 

 rhizome on which arise erect, aerial haulms, 

 usually of annual growth. The rhizome of 

 the common Horse-tail, Equisetum arvense, 

 develops also short tuber - like branches 

 which function as reservoirs of reserve 

 material and hibernating organs (Fig. 420). 

 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 inter- 

 nodes ; they have a central pith-cavity and 

 a peripheral series of smaller air channels. 

 .The collateral vascular bundles form a 

 single circle, as seen in transverse section 

 (Fig. 419). 



The leaves of the Equisetiuae, both in 

 their structure and in the manner of their 

 arrangement on the stem, are also char- 

 acteristically developed. 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 leaves of the successive whorls 

 alternate regularly with each other. 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 them- 

 selves 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 shaped leaves or sporophylls. Like the 



Fu;. 419. Equisetum arvense. Transverse sec- 

 tion through the stem, m, lysigenic medul- 

 lary cavity ; e, endodermis ; d, carinal canals 

 in the collateral bundles ; vl, vallecular 

 cavities ; hp, sclerenchymatous strands in 

 the furrows .and ridges ; eh, tissue of the 

 primary cortex containing chlorophyll ; st, 

 rows of stomata. ( x 11.) 



