170 ROBERT PAULSON ON THE STRUCTURE OF LICHENS. 



(6) Smith, A. Lorrain : A Monograph of the British Lichens, 



pp. 7-23, Part 1. 1918. 



(7) West, G-. S. : Cambridge Botanical HandbooJcs, Algae, vol. i. 



1916. 



Description op Plate 4. 



1. Cladonia digitata. Transverse section of a squamule of 



tlie thallus ; (a) upper cortical layer ; (b) gonidial layer ; 

 (c) medulla; {d.g.) group of daughter gonidia; {m.g.) mature 

 gonidium. x 400. 



2. Cladonia digitata. Isolated gonidium; {c.iv.) ceU wall (light 



area) ; (c) chloroplast, reticulated cytoplasm ; (w) nucleus ; 

 {jp) peripheral body in the centre of a light area, x 4,000. 



Journ. Qiiekeit Microscopical Clnb, Ser. 2, Vol. XIV., No. 86, Novemhcr 1920. 



