PTERIDOrHYTES 143 



is regulated by the guard cells of the stomata, which come 

 together or shrink apart as occasion requires, thus dimin- 

 ishing or enlarging the opening between them. The sto- 

 mata have well been called " automatic gateways " to the 

 system of intercellular passageways. 



One of the peculiarities of ordinary fern leaves is 

 that the vein system branches dichotomously, the forking 

 veins being very conspicuous (Figs. 123-126). Another 

 fern habit is that the leaves in expanding seem to unroll 

 from the base, as though they had been rolled from the 

 apex downward, the apex being in the center of the roll 

 (Fig. 118). This habit is spoken of as circinate, from a 

 word meaning "circle" or "coil," and circinate leaves 

 when unrolling have a crozier-like tip. The arrangement 

 of leaves in bud is called vernation ("spring condition"), 

 and therefore the Ferns are said to have circinate verna- 

 tion. The combination of dichotomous venation and cir- 

 cinate vernation is very characteristic of Ferns. 



80. Sporangia. — Among Thallophytes sporangia are usu- 

 ally simple, mostly consisting of a single mother cell ; among 

 Bryophytes simple sporangia do not exist, and in connec- 

 tion with the usually complex capsule of the sporogonium 

 the name is dropped ; but among Pteridophytes distinct 

 sporangia again appear. They are not simple mother cells, 

 but many-celled bodies. Their structure varies in different 

 groups of Pteridophytes, but those of ordinary Ferns may 

 be taken as an illustration. 



The sporangia are borne by the leaves, generally upon 

 the under surface, and are usually closely associated with 

 the veins and organized into groups t>f definite form, known 

 as son'. A sorus may be round or elongated, and is usually 

 covered by a delicate flap (indusium) which arises from the 

 epidermis (Figs. 118, 123, 124). Occasionally the sori are 

 extended along the under surface of the margin of the leaf, 

 as in maidenhair fern {Adiantum), and the common brake 

 (Pteris), in which case they are protected by the inrolled 



