280 
Proceedings of the Royal Society 
Vast importance lias been attached to the cellulose wall, as an 
assumed characteristic of plants, yet not only the cyst of a Myxo- 
mycete, but that of an Amoeba, is now known to consist of cellulose. 
How is this cellulose wall to be accounted fori Why should the 
resting phase possess a cyst 1 What is the physiological rationale 
of this morphological characteristic of the resting phase 1 
Contracting muscle evolves carbonic acid and water, with evolu- 
tion of heat j the quantity of heat and water products evolved 
diminishes as contractile activity diminishes ; and this physiological 
common-place must hold true of every contracting cell, ciliated or 
amoeboid. But contractility implies waste of formed materials, 
diminution of contractility therefore implies diminution of this dis- 
integration of matter and dissipation of energy, of this combustion 
which we term waste. Cessation of contractility, therefore, involves 
cessation of the combustion of some product — of some fuel which 
was formerly required to maintain the process. The cellulose 
wall ivhich appcai^s on the assumption of the quiescent state is thus 
the equivalent of the carbonic acid and water which were being 
formed and excreted during the sioie of contraction. Being no 
longer required as fuel, it becomes itself thrown out as a waste pro- 
duct — which simply by reason of its chemical and physical proper- 
ties — its insolubility and coherence — acquires at once its morpho- 
logical permanence and its protective use.* 
The applicability of this physiological conception to a new series 
of problems can here only be briefly hinted at. Without more than 
mentioning the discovery of Durin as to the formation of cellulose 
from cane sugar, f it may be briefly pointed out (1) that the occurrence 
of cellulose in Ascidians, or in pathological cases in the human brain, 
&c., is by no means unintelligible — the difflculty is rather the 
reverse — to explain why it is not invariably present in resting cells. 
These are never destitute of external intercellular substance, and the 
A vivid confirmation of the preceding theory of the origin of the cellulose 
wall has been suggested to me since the reading of this paper by my friend Dr 
Milne Murray, who reminds me that a quiescent muscle, instead of evolving 
carbonic acid and water, produces an enormous store of muscle-sugar or inosite, 
and that this is an isomer of cellulose, CgHioO^. The same conception may 
throw light upon the physiological chemistry of other carbohydrates, such as 
glycogen, starch, &c. 
t Ann. Sci. Nat. BoL, 1877. 
