34 ON GROWTH AND EXTENSION IN THE VEGETABLE KINGDOM. 



roots pierce through and split the tubs of our green-houses ; and 

 in quarries we see them at the least forcing up the soil from 

 between the rocks. 



From the first period of the mucilaginous fluid in which they 

 are generated, the cells like the other parts of plants are con- 

 nected together by a kind of glue. Long immersion in water 

 will dissolve this glue, as will also boiling in water, or nitric 

 acid, or the mixture, discovered by Professor Schultz, of muriate 

 of potash and nitric acid, which will separate the strongest woody 

 cells, upon vvhich nothing else will have any effect. The pre- 

 paration of flax takes place by soaking in water, which dissolves 

 the glue, so tliat all the looser cells and vessels easily separate 

 from the woody fibres ; by boiling, leguminous fruits are so 

 softened tliat the cells can be readily separated ; and in the same 

 manner all the tenderer parts readily separate after steeping in 

 nitric acid. In a natural state this glue dissolves of itself in 

 the beri'ies of many plants. This matter has often been called 

 also intercellular substance ; but it is distinguished from it in 

 this, that it is always present in the living plant, although not 

 always visible ; whilst the intercellular substance, as distinguished 

 from the adjoining cells, is often wanting. 



We must carefully distinguish from the cambium the sap which 

 in spring will flow in great profusion on boring the stems of 

 several trees, especially beeches, and some species of maple. 

 This comes from the outer layers of wood, at a time when the 

 bark still adheres strongly to the wood : it can be only obtained 

 by incision or by boring ; that is, by piercing the receptacles 

 in which it is contained. It will begin to flow from the lower 

 part of the trunk at a time when the upper part and the 

 branches when pierced will remain quite dry. It flows much 

 more in warm than in cold weather; and, lastly, it is so much 

 divided by distribution in the smaller twigs and leaves that it 

 will scarcely flow from them. Indeed, in cool weather, the flow 

 will cease in tlie upper part of the stem, whilst it will continue 

 below. I have often myself had an opportunity of observing 

 this fact, which is, moreover, well known to gardeners. It is the 

 stimulus of heat which excites the vessels and causes the rise of 

 the sap, which issues from the wound because it cannot spread 

 in the upper parts. As the bark is still closely adhering to the 

 wood when the sap flows, the latter cannot contribute to the for- 

 mation of the new layer of wood : the sap is not cambium. 



The cell consists externally of a very fine membrane (the cel- 

 lulose), which is lined on the inside with the cellular deposit, 

 arranged often in several layers. In certain places only the 

 external membrane remains uncovered by it, or rather it does 

 not form there ; and these places remain clear and transparent, 



