122 MANUAL OF BOTANY 



surrounding the stem. They are of two kmds ; some are ordinary 

 foUage or scale leaves ; others bear sporangia, and are known as 

 sporophylls, as in the higher plants. In the ferns there is 

 usually no difference between the two, but in the other groups 

 the sporophylls are often much modified. In the Equisetina; 

 and some of the Lycopodinse they are collected together at the 

 apices of special shoots, forming cones or flowers. 



The sporophylls bear the sporangia usually in clusters, 

 known as sori, which may be exposed to the air, or variously 

 covered in for purposes of protection. In some cases the 

 sporangia arise directly from the stem in the axil of the sporo- 

 phyll. In the ferns and horsetails the sporangia are on the under 

 or dorsal surface ; in the club-mosses on the upper one. 



The primary roots are shortlived, soon being replaced by 

 adventitious secondary ones. Salvinia has no primary or 

 secondary roots, nor, so far as we know, have some species of 

 the isosporous ferns and some of the club-mosses. 



A peculiar feature makes its appearance in the Pteridophyta, 

 which persists throughout all the higher plants. This is the 

 production by the sporophyte of two kinds of spore, each of which 

 -gives rise to its appropriate gametoph_\ te. The two kinds of 

 spore differ from each other chiefly in size, some, called 

 microsjwres, being very small, and others, 7nacrospores, much 

 larger. Each is produced in its appropriate sporangium, so that 

 we distinguish micro- and macrosijoycmgia. The peculiarity of 

 producing both kinds is called lieterosjjory ; we sj)eak thus of 

 isosjjoi'ous and heterosporous Vascular Cryptogams. 



It some cases all the sporangia of a sorus are closely attached 

 together as if coherent. They are so developed, never being 

 free from each other. The resulting body appears like a multi- 

 locular sporangium, but it is better to regard it as compound 

 and call it a sijnangiuni. 



A similar multilocular appearance is afforded by the sporangia 

 of Isoetes, but in this case it is due to bands of the archesporial 

 tissue becoming sterile, and crossing the sporangium in the form 

 of a number of trabeculae. 



Sporangia are classed in two categories, according to the 

 manner of their development. In some plants, as in many 

 ferns, they arise each from a single superficial cell. These are 

 known as leptosimrangiate forms. In others, as in the Marattias 

 and Isoetes, the sporangia arise from a group of cells of the 

 superficial and sometimes deeper layers as well. These forms are 

 called easporangiate. 



