FORMATION OF WOOD. 7 



albumen or white of egg, are known as albuminoid, and, from the 

 readiness with which they undergo chemical change or decompo- 

 sition, as proteids. Being the substance out of which all plant- 

 structures originate, the sole constituent of the first germs of all 

 living beings, it is known as protoplasm, from the Greek protos, 

 first, plasma, formed matter. 



Any collection of similar cells or modifications of cells having a 

 common origin and obeying a common law of growth is known as 

 a tissue. These young cells at the apex of a stem, of nearly 

 uniform size, and that extremely minute, with their delicate, as 

 yet unaltered, cell-walls filled with protoplasm, form an embryonic 

 tissue, one, that is, which will undergo change. Its uniform 

 character causes it to be termed undifferentiated, while the various 

 kinds of tissue to which by different changes it gives rise are 

 known in contradistinction as permanent tissues. One change to 

 which any cell is liable so long as it contains protoplasm is 

 division into two, a partition wall of cellulose forming across it. 

 The formation of this solid wall from material in solution in the 

 protoplasm, and a correlative power, which, as we shall see, the 

 living plant possesses, of dissolving a cell-wall, illustrate that 

 interchangeability of sugar and cellulose of which we have 

 spoken. A tissue the cells of which undergo division is termed 

 merismatic or meristem, from the Greek merisma, division ; so that 

 the embryonic tissue at the apex of a stem is known as apical 

 meristem. 



Although its cells are all embryonic, they nevertheless at a 

 very early stage commonly present such a degree of differentia- 

 tion as to make it possible to distinguish three well-defined 

 rudimentary tissue-systems (Fig. 2). First, there is a single 

 layer of cells on the outside of the growing-point, with thickened 

 outer walls and undergoing division only in planes perpendicular 

 to the surface. If we trace this layer backwards down the 

 surface of the shoot below its apex we shall find it continuous 

 with similar cells which have lost their protoplasm and have even 

 thicker outer walls. As this outer layer of permanent tissue is 

 called the epidermis, from the Greek epi, upon, derma, skin, the 



