512 



HUTCHINSON'S POPULAR BOTANY 



FIG. 653. PILLWOBT. 



One of the microspores. 



and gradually unroll as they grow to their full 

 size. 



Marsilea, which grows in similar situations to 

 Pilularia, but does not occur in this country, has 

 a quatrefoil leaf, the segments of which fold 

 together towards night and expand again in the 

 morning, thus agreeing in its sensitiveness to light 

 with the leaves of Oxalis and other trefoils among 

 Phanerogams. 



The Class Equisetinae, or Horsetails, consists of 

 the order Equisetacese and a single genus, Equi- 

 setum, of erect, hollow-stemmed, jointed, and leaf- 

 less plants. The leaves are represented by the 

 teeth of the sheath in which each node terminates. 

 The branches, where present, are jointed, but, unlike the stem, are solid, 

 and spring from the base of the sheath. The stems are ridged and grooved 

 longitudinally, each species having a characteristic number and form of 

 ridges, and the sheath-teeth correspond with them. In some species there 

 are barren and fertile stems, the latter being without branches and almost 

 or entirely devoid of chlorophyll. Silica is deposited abundantly in the cell- 

 walls of the cuticle, and in consequence several species have long been used 

 under the name of Dutch Rushes for scouring and, polishing metal. So 

 abundant is this mineral that all the vegetable matter may be burnt out 

 without affecting the form of the structure. Stems and branches alike are 

 provided with stomates and chlorophyll, and can carry on the functions 

 of the absent leaves. 



The fertile stems bear at their extremity a cone-like spike of sporange- 

 bearing discs. These discs or scales are the sporophylls ; they are many- 

 sided (usually hexagonal), supported on a central foot-stalk, and bear on the 



under side from five to ten sporanges. 

 The sporophylls are arranged in whorls 

 corresponding with the sheaths, and prob- 

 ably, like them, are modified leaves. Be- 

 tween the uppermost developed sheath 

 and the lowest whorl of sporophylls there 

 ,. is an undeveloped sheath forming an in- 



\ s ^ m_& ; y volucre to the fruit-spike. The sporange 



^*^^^B H^^**" 1 ^ has no annulus, and opens by a longi- 



^^^^ tudinal fissure to set free the spherical 



spores, which differ from those of ferns 

 in having four coats, the outer of which 

 splits up spirally into four strips with 

 broader ends, known as elaters (fig. 654). 

 These elaters are highly hygroscopic, and 



^/ \^/ 



FIG. 654. HORSETAIL (Equisetum). 



(a) Spore with elaters coiled around it. (6) The 



same with the elaters extended. 



