172 PRACTICAL BOTANY. 



ii. A layer of small, compressed cells. 



iii. A layer of thin-walled cells, elongated radially : 

 this is the Tapetum, which is here persistent until the 

 spores are ripe. 



Surrounded by the wall will be found 



(c.) Spores of two sorts 



i. Microspores of relatively small size; these will 

 be found in large numbers in certain sporangia, which 

 will accordingly be recognised as Microsporangia. 

 When ripe they may be still seen to cohere in groups 

 of four : each spore is a single cell with a brown wall. 



ii. Macro spores of relatively large size : four only 

 of these will be found enclosed in the sporangium, which 

 is accordingly termed a Macrosporangium. Each 

 spore consists of a thick wall, with numerous external 

 projections, surrounding a large cavity filled with 

 protoplasm. 



The development of tlie sporangium may be traced in longi- 

 tudinal sections of sporangium-forming cones which have been 

 hardened in alcohol, or better, in picric acid and then in alcohol ; 

 mount in glycerine. The following points in the process of 

 development may be observed. The sporangium is first seen 

 as a swelling of a group of cells at the surface of the apical cone, 

 above the leaf in the axil of which it appears : thus the sporan- 

 gium is not borne on the leat as in Lycopodium, but springs 

 from the tissues of the axis. A central row of cells grows more 

 strongly than the rest, and the outermost cell but one of this 

 series may be recognised as the Archesporium. The outermost 

 cell divides to form part of the two outer layers of the wall of 

 the sporangium. The archesporium also divides to form a mass 

 of tissue, of which the peripheral layer becomes the Tapetum 

 (the basal part of the tapetum is however derived from the 

 adjoining tissue). The central part of the tissue derived from 

 the archesporium forms the spores ; each spore-mother-cell sepa- 

 rates from its neighbours, and divides into four cells. If the spo- 



