ON THE PHYSICAL STRUCTURE OF PLANTS. 
45 
ferent layers of wood into angular sections. Their use is to connect 
the concentric layers of wood, and perhaps to admit or allow the passage 
of fluids from the circumference to the centre of the stem. 
Vegetable membrane varies not only in consistence, but in durability. 
The lower orders of plants are mostly cellular, that is, composed entirely 
of simple cells, without fibres or elongated vessels to give them either 
tenacity or durability. Their growth is rapid, and they as quickly dis¬ 
appear by decomposition in the open air. But the membranes of vascular 
plants become woody, and, when the cells are filled with concreted sap, 
remarkably durable, more especially when converted to use, and kept 
free from the action of the weather; even some of the cellulares, as the 
fungi, for instance, if gradually dried and kept free from moisture, will 
retain their form for a great number of years; and many of the musci, if 
kept in a dry and dormant state for scores of years, will recover vitality 
as soon as they are moistened with water. This is only mentioned as a 
proof of the durability of vegetable membrane, even in the lowest grades 
of plants. 
When we turn to the vascular dicotyledonous tribes, we find many 
instances of extreme ponderosity, hardness, and durability ; so weighty 
that it will not swim, as lignum vitce ( Guaiacum officinale ), and which 
maybe instanced also as an example of both hardness and durability, even 
when buried in the earth. Both box and yew timber are remarkable 
for their closeness of grain and lasting properties, both while the trees 
are standing, as after they are felled. Oak timber is very durable, and 
so is that of many of the coniferce. Solidity is acquired by the close¬ 
ness of the cellular and vascular structure, as is exemplified in the sorts 
of wood above named, and if the fabric be well filled with coagulated 
juice, ponderosity will be added. 
But another question has been mooted; it is, whether the durability 
depends on the density of the grain, or on the qualities of the sap it 
contains ? The durability of pine timber evidently depends on the 
slowly evaporative and preservative quality of the sap. The same, 
perhaps, may be said of oak, not, indeed, from the resinous character of 
the sap, but from the styptic and tanning principles contained in the 
heart-wood itself. 
Experiments have been made on different kinds of timber, by sub- 
mitting small portions of them to the action of an acid menstruum. 
The least durable is soonest decomposed; the parenchymous or cel¬ 
lular parts are first dissolved, and the fibrous tissue remains, and is 
last destroyed. By this means beautiful skeletons of slips of wood, 
seed-vessels, and leaves, are readily obtained. It has been said that a 
