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. Each 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 pro f oplast 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 



