506 



HUTCHINSON'S POPULAR BOTANY 



FIG. 645, CUP-SHAPED IK- 



DUSITJM OF FILMY FERX (Hy- 



menophyllum), and the same 



in section. 



consist of a single layer of cells. A longitudinal 

 row of these cells, known collectively as the 

 annulus, have thickened borders. From two to 

 four others, known as lip-cells, have lignified walls ; 

 it is between these lip-cells that the dehiscence 

 or rupture of the sporange begins. This is 

 brought about by the drying and consequent un- 

 equal contraction of the cells of the annulus, 

 which pulls the lip-cells apart: following their 

 separation the sporange splits across and scatters 

 the spores. The annulus does not form, a com- 

 plete ring in this order. Frequently from the 



foot-stalk of the sporange in Polypodiaceae there is a hair-like outgrowth 

 a paraphyse. There are normally sixty-four spores in each sporange of 

 this order, produced in the usual way by division of the archespore into 

 mother-cells and subdivision of each of these into four spores. 



In the Family HymenophylleaB (Filmy Ferns) there is usually a creep- 

 ing stem, and the fronds are very thin and translucent, the mesophyll 

 consisting of one layer of cells only. The indusium is cup-shaped (fig. 645). 

 The fertile vein projects beyond the edge of the frond, and a prolongation 

 of it called the columel extends into the centre of the indusium, where the 

 sporanges are borne spirally upon it. Instead of being seated on a foot- 

 stalk as in Polypodiacese, they are attached to the columel by one of the 

 two convex faces. The annulus is complete. The spore undergoes division 

 into three cells before the rupture of the exospore, two of which soon cease 

 to develop, but the third increases greatly in length, divides transversely, 

 and puts out thread-like lateral shoots, from which flat prothallia are 

 produced. As will be seen later, this form of prothallium approximates to 

 the protoneme of Mosses, and as the order contains the simplest of the ferns 

 it may be that it marks a stage in their evolution from the Mosses. The 

 antherids will be found about the middle of these filaments, the archegones 

 at the extremity. The phenomenon of 

 apogamy the substitution of a vegetative 

 for a sexual mode of reproduction occurs 

 frequently in this order, and it is thought 

 may be quite usual in certain species. 

 Little foot-stalks (sterigmas] produced from 

 the prothallium bear bulbils of a few cells 

 which slowly germinate and grow into 

 sporophytes without any process of fertili- 

 zation. All the species in this order are 

 of exceedingly delicate texture, and can 

 only endure a moist warm atmosphere. 

 The fronds are without stomata. 



FIG. 646. ANTHEROZOIDS. 



(a) Antherozoid in mother-cell. (6) Antherozoids 

 escaped from cells. 



