214 PLANT LIFE. 



are often produced in great profusion, especially by the fungi, 

 the mosses, the ferns, and the seed plants. 



308. Form and structure. — Their form is exceedingly 

 various. Many are spherical or ovoid, while some are cylin- 

 drical or even needle-shaped (figs. 213, 228, 271). Irregular 

 forms, also, are not uncommon. 



In structure spores are usually only single cells, specialized. 

 Each is a nucleated mass of protoplasm surrounded by 

 a cell-wall which may be either thin or thick, according 

 as the spore is destined to immediate growth, or, as a 

 resting spore, to endure for a time unfavorable conditions. 

 ^^ In some cases the wall of even the 



/^« £fa§mW&K thin-walled spores has two layers, a 

 condition which is almost universal 

 among resting spores. When the 

 wall is so differentiated the inner 

 layer is delicate, rarely thickened, 

 extensible, and composed of more or 

 \SpIW ^®§^&j * ess unait ered cellulose. The outer 

 v ^w^^gW3^g^w'^ layer is often irregularly thickened, 

 ^^ffiSI^^ s0 tnat * ts sur ^ ace i s covered with 

 fig. 2 o9.-Section of a mature ridges, warts, spines, or bosses of 

 rt£«^r£^2S; various sorts (figs. 210, 248, 271, 

 rtTl ; rhetrnd W ft ; he399)- » » brittle, as compared 

 ^^^%T^iMth the inner coat, and is usually 

 nfSKr^^r-S more or less altered in composition 

 Sachs " from its original cellulose nature. 



A third layer (the epispore) is sometimes present, but this 

 is not produced by the cell which it surrounds. It is added 

 from the outside, being derived from the protoplasm sur- 

 rounding the spores after they are formed * (fig. 209). This 

 form of spore is common among the fern allies. 



* This protoplasm often comes from the disorganization of some of the 

 cells around the chamber in which the spores lie. 



