306 PHILOSOPHICAL NOTES ON 



Gustave Thuret (" Etudes Phycologiques," 1878, plates XI., 

 XV., XVI.,) gives figures of three species of Fucus ; serratus L., 

 vesciculosus L., and platy carpus, Thur. The whole frond of 

 all three is covered on both sides with conceptacles or spore-sacs. 

 But it is only towards the points of the fronds that these con- 

 ceptacles are fertile. In the remaining portion of the frond they 

 are abortive, indicating most probably ^that in antecedent forms the 

 whole frond was covered ^y\i\l fertile conceptacles. The sections 



Fig. 130. Cross section of the frond of Fucus platycarpus, showing the 

 conceptacles opening on both sides of the frond (Thuret, " Etudes Phycolog," 

 pi. 17). 



given by Thuret of the fertile parts (Fig. 130) show the spore 

 sacs on both sides of the frond, some mature and open, and others 

 progressing towards maturity and closed. 



Now Penzig in his " Studj sugli Agrumi," p. 52, has shown 

 that the oil-glands of the Citrus are on both sides of the leaf. 

 He further shows that the oil-gland is a sac filled with cellules. 

 At a certain stage, as already stated, the central cellules dissolve, 

 and turn into essential oil, while the outer ones remain as a lining 

 to the sac. The mouth of this sac or oil-cell is closed by a layer 

 of epidermic cells, which in the orange peel with a little pressure 

 bursts, and the oil squirts out. 



I confess I do not see anything preposterous in considering the 

 essential oil-glands of the Citrus as possible degenerations of 

 conceptacles in some fucus-like remote ancestor. 



Judging from the Fucus of to-day, I should say the fertile 

 conceptacles of its ancestor were scattered all over the frond on 

 both sides. But certain portions of the frond, in subsequent 

 generations, having been specialized as conceptacle bearers, the 

 remainder of the frond retained traces of them as atrophied 

 remnants. 



