70 ELEMENTS OF PLANT ANATOMY. 
cells becoming (or growing) prosenchymatic and assuming the 
power of division. In the same way it is possible for any cell, 
as long as it retains its protoplasmic contents and living energy, 
to change from a lasting to a meristematic cell, or as it is some- 
times expressed, “take on the power of wall formation” by 
which new cells are formed. Owing to this fact the plant has 
power to form organs’or aggregates of cells for special functions, 
which cannot properly be included in any of the three systems 
just described. Therefore when following this classification of 
tissues we are obliged to add certain classes of cells as forming 
special organs not included in these systems. These are de- 
scribed in the next section. 
5. Organs of Secretion and Milk Tubes. 
Certain parts of the plant are known to secrete substances 
which are of no further use in its metabolistic processes but may 
be of service in other ways. These parts are known as secretory 
organs or glands. Glands may be defined as all sharply differ- 
entiated parts of tissue which are entirely or mostly filled with 
these secretions. This does not include the milk tubes, which, 
owing to several peculiarities, are classed by themselves. 
Glands are of two classes, those which excrete their contents, 
and those which retain them. ‘These are again subdivided as 
is shown in the following table : : 
External. 
Internal. 
Tubes and tubular ducts. 
Intercellular channels. 
1. Those excreting contents 
Glands. 
2. Those holding contents 
EXTERNAL GLANDS. 
If wax and cutin can be considered secretions, the whole 
epidermal system might be included under those glands of the 
first class which excrete their contents externally. This, how- 
ever, is hardly practicable, and it is customary to consider the 
