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 

 ' ',^ thin-walled spores has two layers, a 

 ^ <A condition which is almost universal 

 •$ among resting spores. AVhen 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. 209.-Section of a mature ridges, warts, spincs, or bosscs of 



271, 



r^p 



spor^ol Pnulariaj,M«Hjcra.^^^-^^^^ g^j.^^ ^^^^^ ^^^^ ^_^g 



It is brittle, as compared 



/;, the cavity of the cell (contents 



not shown); it, the true wall; /' orirv \ 



the first, c the second, d the j99/- 



Ss'rp;pma^V,?'Tand5with the inner coat, and is usually 



have a prismatic structure. Mae- i i j. j ■ • j_ ■ 



nified about 50 diam. - Aftir niorc Or Icss altered m 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 pirotoplasm sur- 

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

 form of spore is common among the fern allies. 



* Thi.s protoplasm often conies from the disorganization of some of the 

 cells arottiul the chamljcr in which the sptores lie. 



