BOTANICAL SUBJECTS. 279 



being controlled by two cells at the opening, the hygrometric 

 state of the air within may be modified by the transpiration of the 

 inner cells. 



From all this it would appear that the distinction, which 

 microscopists and histologists make between trichomes, as 

 belonging to the epidermis, and other structures, is only one of 

 words. 



A little consideration 'will show that all the lower fungi, 

 sporangia, antheridia, &c,, &c., are indistinguishable from 

 trichomes, except that they give forth reproductive cells, while the 

 trichomes, or hairs proper, have this function aborted. 



When land plants evolved out of water plants, the outer layer 

 of the mass of cells we call epidermis became atrophied, and 

 functioned as a protective covering to the softer tissues beneath, so 

 that the latter might still carry on a ^M«5?-water-Hfe. Then 

 outgrowths from this layer further protected the plant from 

 cold, &c. by adding a hairy covering like that of animals. 



But all this did not suppress in the trichomes the potentiality 

 of reversion to ancestral functions, whenever conditions became 

 favourable. Microscopic fungi appear to be nothing but 

 trichomes, Avith vegetative and reproductive powers, though both 

 are of a rudimentary character. 



Curiously enough, one cannot look through the chawings of 

 G. Thuret's " Etudes Phycologiques " (1878) without seeing, at a 

 glance, that the most imjjortant organs of ses.\veeds ■ -antberidia and 

 spores — corresponding to stamens and ovules in pha^nogams — have 

 no other but the structure of trichomes. Their emergence too is 

 from the superficial layer of cells of the frond, which would 

 correspond with the epidermis in phsenogams.* 



Thuret calls the stems, which bear the antheridia and sf>ores, 

 hairs (polls), and rightly too, for no one could call them anvthing 

 else. These hairs, nevertheless, branch into sexual organs ! 



The antheridia of Fucus serratus, pi. 11 of Thuret's work, are 

 also nothing but sub-divisions of hairs. The hairs interspersed 

 among the spores in pi. 12 are probably only atrophied antheridia, 

 reducing the sexual organs, originally monoecious, into the dioecious 

 form. 



* Vide pis. 9 and 10 of Thuret's work. 



