FILICALES 67 



stems, sheath-like whorls of united leaves and terminal cone-like fruc- 

 tifications. Their bodies contain large amounts of silicon, hence the 

 name scouring rushes. 



In some varieties the fruiting cone is borne on the ordinary stem, in 

 others on a special stem of slightly different form. In the latter the 

 spores are provided with elaters, which, being hygroscopic, coil and un- 

 coil with increase or decrease in the amount of moisture present, thus 

 aiding in the ejection of spores from the sporangia. The number of 

 species is small and included under one genus Equisetum. 



SUBDIVISION III.— FILICALES 



The group Filicales is the largest among the vascular cryptogams 

 and includes all the plants commonly known as Ferns. The main axis 

 of a typical fern is a creeping underground stem or rhizome which at 

 its various nodes bears rootlets below and fronds above. These fronds 

 are highly developed, each being provided with a petiole-like portion 

 called a stipe which is extended into a lamina usually showing a forked 

 venation. Some ferns possess lamina; which are lobed, each lobe being 

 called a pinna. If a pinna be further divided, its divisions are called 

 pinnules. The unfolding of a frond is circinate and it increases in length 

 by apical growth. On the under surface of the laminte, pinnae, or 

 pinnules may be seen small brown patches each of which is called a 

 sorus, and usually covered by a membrane called the indusium. Each 

 sorus consists of a number of sporangia (spore cases) developed from 

 epidermal cells. In some ferns the entire leaf becomes a spore-bearing 

 organ (sporophyll). Most sporangia have a row of cells around the 

 margin, the whole being called the annulus. Each cell of the annulus 

 has a U-shaped thickened cell wall. Water is present within these 

 cells and when it evaporates it pulls the cell walls together, straightening 

 the ring and tearing open the weak side. The annulus then recoils 

 and hurls the spores out of the sporangium. Upon coming in contact 

 with damp earth each spore germinates, producing a green septate 

 filament called a protonema. This later becomes a green heart-shaped 

 body called a prothallus. It develops on its under surface antheridia 

 or male organs and archegonia or female organs as well as numerous 

 rhizoids. Within the antheridia are developed motile sperm, while 

 ova are produced within the archegonia. The many ciliate sperms 

 escape from the antheridia of one prothallus during a wet season and 



