Mr. G. H. K. Tliwaites on the Gonidia of Lichens. 219 



Sarcjassum Wiyhlii. 



Fig. 1. Portion of a branch. 



— 2, 2. Leaves and vesit-les from a young specimen. 



— 3. Raceme of fructification as sometimes seen tern)inating tlio 



brandies. 



— 4. Portion of a raceme in its more compact form. 



— 5. Portion of do. as Been in fig. 3. 



— 6. Vesicle. 4 & 5 magnified. 



Saj'gassum cervicorne. 



Fig. 1. One of the fertile ramuli, and leaf given off at the base of a branch. 



— 2. Leaf from a young plant with vesicles. 



— 3. Do. from towards the upper part of same plant. 



— 4. Vesicle;^. 



— 5. Receptacles as they are developed on one specimen. 



— 6. Do. The last ma«;nified. 



XXV. — On the Gonidia of Lichens. By G. H. K. Thwaites, 

 Lecturer on Botany and Vegetable Physiology at the Bristol 

 Medical School. 



[With a Plate.] 



There appears to have been much uncertainty felt by those who 

 have devoted their attention to the study of the Lichens, as to 

 the real character of those spherical or subspherical green bodies 

 which are characteristic of true Lichens, and to which the name 

 oi gonidia has been given, from the circumstance of their capa- 

 bility of becoming developed into new plants when separated 

 from the parent structure. Every one who has examined care- 

 fully the ihallus of a Lichen under a tolerably high power of the 

 microscope, must have been struck by the peculiar appearance of 

 the gonidia^ as compared with ordinary cellular structure : — the 

 frequent irregularity in their form — their want of correspondence 

 in size — their slight attachment to each other, or to the filamen- 

 tous tissue surrounding them, and their aggregation in certain 

 parts of the structure — must have taken the attention of any 

 observer who has been much accustomed to the examination of 

 vegetable structures. These peculiarities indeed gave rise to a 

 strong desire on my part to ascertain the real character of ^omWz«, 

 and after examining a great number of species, both of true 

 Lichens and of the genus Collema, and plants allied to it, I am 

 able with confidence to state what is the true character oi gonidia. 

 It is pretty generally known that the thallus of Collema consists 

 of a number of moniliform filaments, and also of delicate anasto- 

 mosing cylindrical filaments immersed in a more or less firm ge- 

 latine. When examined more carefully the structure is found 

 to consist of numerous Nostoc-like vesicles closely cohering, and 

 among which ramify the anastomosing filaments. The cellular 



