^■4 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 

 ' ') JL[\ A -'Jf/fr 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 

 less unaltered cellulose. The outer 

 layer is often irregularly thickened, 

 so that its surface is covered with 



Fig. 2 oo.-Section of a mature ria ^es, WartS, Spines, Or boSSCS of 



rt"el^of^fdf(co& various sorts (figs. 210, 248, 271, 

 u;r!-rVu; e t tcZ,.^'u 1 e'399)- " is brittle, as compared 

 S^pma^tT^ndiwith the inner coat, and is usually 

 ^ V ed a To«t ati so St diaS re -fe-more or less altered in composition 

 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 

 tells around the chamber in which the spores lie. 



