2 ELEMENTS OF PLANT ANATOMY. 
contents were the only necessary element. This is shown by 
the fact that the wall is a product of the contents, and that 
at certain periods of the plant’s life the cell may exist without 
it. Discoveries of this kind gave rise to an entirely different 
conception of the nature of the plant-cell. It is now known 
that this, in its simplest or least differentiated condition, con- 
sists of a small portion of the viscous. liquid known as proto- 
plasm, in which, under ordinary magnification, no structures 
are visible. It is in this general sense that Reinke defines a 
plant cell as follows: —“ An individualized, not farther divisible 
structure, consisting of or containing protoplasm which either 
shows life processes or has shown them.” 
In studying the anatomy of a plant cell it will be necessary 
to consider one in its ordinary condition of development, that 
is, aS an element of any plant, differentiated sufficiently to 
perform the ordinary functions of plant cells. Such cells are 
usually considered as consisting of two parts, wall and contents, 
or as it is frequently stated, wall and protoplasm, the latter 
including a nucleus and one or more vacuoles. Before taking 
up the study of these parts separately, it may be well to examine 
the cell as a whole, in reference to several features, namely : — 
size, form, mechanical and physiological principles, and finally 
to discuss briefly certain theories concerning organized structures 
in general. , 
By far the greater number of plant cells are microscopic, 
but they vary greatly in size. The smallest occur among the 
organisms known as bacteria. Some of these are spherical in 
form and measure from seven-tenths to one micro-millimeter 
in diameter. Among the largest plant cells may be mentioned 
those forming the internodes of the stem in the group Characeae, 
where the cell is often several inches long. Pollen tubes, which 
are also single cells, often attain a length of several inches, and 
the milk tubes of certain plants are said to reach even a greater 
length than this. 
