SYMBIOSIS IN ANIMALS AND PLANTS. 
ADDRESS TO SECTION I (PHYSIOLOGY) BY 
GEORGE H. F. NUTTALL, M.D., P#.D., Sc.D., F.R.S., 
Quick Professor and Director of the Molteno Institute for Research in 
Parasitology, University of Cambridge, 
PRESIDENT OF THE SECTION. 
CONTENTS. PAGE 
Introduction . : F “ - N 5 : 197 
I, Symbiosis in Plants :— 
(1) Lichens . : 4 : 5 . ; 6 LOT 
(2) Root-nodules of Leguminous Plants 3 s A ; . 199 
(3) Significance of ei heac in various Plants : : C 200 
(a) Orchids . : F : . F - 200 
(b) Origin of Tubers i in, various Planta ‘ e P . L202, 
(c) Ericacez 5 - - H 3 : ; 2\n 203 
(d) Club-mosses and Ferns . : : : : f > 203 
II. Symbiosis in Animals :— 
(1) Alge as Symbionts in various Animals : : : é . 203 
(2) Symbiosis i in Insects f - 206 
(3) Micro-organisms in relation to pee in rntrials é - 9 209 
(a) Luminescence due to Parasitic Organisms 210 
(b) Luminescence due to Symbionts in Insects, Cephalopods, 
Tunicata (Pyrosomide) and Fish ‘ : : 210 
Portier’s Hypothesis < A A 2 i A . : 212 
Conclusion . . 2 “ : . - ; . - - 213 
Introduction. 
THE subject of symbiosis has been chosen for this address because of its 
broad biological interest, an interest that appeals equally to the physio- 
logist, pathologist, and parasitologist. It is, moreover, a subject upon 
which much work has been done of recent years in different countries, 
and this seems a fitting occasion upon which to give a brief summary 
of what is known to-day, especially since the literature relating to 
symbiosis is largely foreign, somewhat scattered and _ relatively 
inaccessible. 
I. Symbiosis in Plants. 
(1) Lichens. 
It is well known to botanists that the vegetative body (thallus) of 
lichen plants consists of two distinct organisms, a fungus and an alga. 
The alga, individual elements of which are called ‘ gonidia,’ is either 
scattered throughout the thallus or, as in most cases, it forms a well- 
defined layer beneath the surface of the thallus. The view that lichens 
