42, VEGETABLE PHYSIOLOGY 
prepares its own defensive mechanism. In most cases 
the protoplast is always clothed by a cell-wall, the forma- 
tion of every new cell being completed at once by the 
membrane which is formed between the two halves as soon as 
the protoplast has divided into two. This is particularly 
noticeable in cases where a cell-complex or community forms 
the plant-body. Hach protoplast thus continually secures 
for itself a chamber to dwell in, the walls of which at first, 
at any rate, are probably all alike, though one is younger 
than the rest. We may consequently recognise in the 
cell-wall an exoskeleton formed for itself by the individual 
protoplast, which may or may not undergo subsequent 
modification. 
In the case of a large plant consisting of innumerable 
protoplasts, the cell-walls of the separate units are united 
together in various ways, and to a different extent in differ- 
ent individuals. The resulting network constitutes at first 
the skeleton of the whole plant. The modification of the 
cell-wall, which was unnecessary in the case of a solitary 
protoplast, becomes imperative as soon as the needs of a 
large community are established, and’secondary differentia- 
tions of such cell-walls result, the alterations being due, like 
the original formation, to the activity of the protoplasts. 
Not only are the walls changed in substance and increased 
in thickness after they are formed, but the protoplast itself 
frequently alters the shape of the cavity it has constructed 
for itself, and consequently its own form, by irregularities 
of subsequent growth, which we shall discuss in more detail 
later. The skeleton of the plant comprises therefore not 
merely the hard tissues which will survive maceration and 
desiccation, those coarser structures evidently set apart for 
protection and support, but also all the delicate cell-walls 
which form the cavities in which the protoplasts are living. 
We may indeed discriminate between the skeleton of the 
individual protoplast and that of the large community of 
which it forms a part. 
The skeleton of a large plant such as a tree increases in 
