8i 



ing a coil extends vertically upward so that the free end projects 

 somewhat above the surface of the thallus ; the entire structure is 

 known as the carpogone ; the spiral coil as the ascogone and the 

 free cone-like end as the trichogyne. The carpogone is supposed 

 to be the female reproductive organ. The male fertilizing element 

 (spermatium) escapes from the male receptacle (spermagonium) and 

 is carried to the trichogyne by water-currents (rain, dew, rarely run- 

 ning water), whereupon it adheres to the conical projection for the 

 purpose of fertilization. As soon as fertilization is completed the 

 ascogone begins to develop into the apothecium ; the ascogone en- 

 larges and becomes flattened, forming the sub-hymenial layer, from 

 which the spore-sacs develop ; the paraphyses arise from a distinct 

 hyphal tissue lying near the ascogenous tissue. Stahl's observations 

 have been seconded by Sturgis. The latter author recognizes two 

 forms of the sexual process, the one "characterized by the transfor- 

 mation of the spermagonium into an apothecium after the fertilization 

 of the carpogonium," by which he doubtless means that the car- 

 pogonium takes its origin in the receptacle of the spermatium; the 

 other sexual method is "characterized by a complete separation 

 throughout their entire course of development, of spermagonia and 

 apothecia." 



Some authors also speak of monoecious and dioecious lichens, 

 in the former both sexual organs (spermagonia and ascogonia) oc- 

 curring on the same plant. Only a few species are supposed to be 

 dioecious (example: Efhcbe ■pubescens.) 



V 



VI. MECHANICAL ADAPTATIONS. 



On account of the great length of certain fruticose thalli and the 

 great surface expansion of some foliose thalli (several feet or more) 

 it is reasonable to suppose that there must be special mechanical 

 tissues to resist the forces continually acting upon them. The forces 

 to be resisted are either lateral or longitudinal. Longitudinal ten- 

 sions may be either pulling or pushing (supporting). It must, how- 

 ever, be remembered that no mechanical tissue is wholly adapted to 

 resist one given tension, that is, while a tissue is especially adapted 

 to resist pulling tensions it nevertheless also resists lateral tensions as 

 well as longitudinal compression (example : Usnea barbata) . 



In lichens the mechanical tissue is in most cases a product of the 

 fungal symbiont. Besides its mechanical function it may serve as a 



