Permeability 
The small granules occurring throughout the 
in some cases tiny non-living inclusions and 
plastids or other living bodies. Chemically they may be composed 
of fat, glycogen, protein, etc., as shown by microchemical tests. 
Chambers divides them into microsomes and macrosomes, the former 
being very minute and considerably less than i /x in diameter, while 
the macrosomes vary from 2 to 4 /x in diameter and are circular, 
oval or polygonal in shape. The macrosomes are the alveolar spheres 
of Wilson (1899), with whom Chambers disagrees as regards the 
complete gradation from macrosomes to microsomes. Chambers 
regards the two sets of bodies as quite distinct; the macrosomes 
are the most easily injured, and the microsomes are the most re¬ 
sistant, of all cell structures. Into this controversy we need not 
enter here, nor yet into the question to what extent the micro¬ 
somes are identical with mitochondria. Those interested should 
consult the very numerous papers on this subject recently published 
by Guilliermond (1916-1921). 
Whereas in animal cells the protoplasm frequently occupies the 
whole of the cell space, in plants this is the case only with 
meristematic cells. As the cell passes out of the meristematic con¬ 
dition small inclusions of less viscous consistency become obvious 
in the general mass of the cytoplasm. These are the vacuoles. As 
the cells grow older the vacuoles become larger and ultimately fuse 
into one large vacuole occupying the greater part of the cell cavity. 
Strands of cytoplasm traverse this vacuole and may even in the 
middle of the vacuole form a mass enclosing the nucleus. In the 
oldest cells which are still living even these strands may disappear 
and the cytoplasm is limited to a scarcely perceptible layer sur¬ 
rounding the vacuole. In this case the nucleus lies in this lining 
layer of cytoplasm; it is never in direct contact with the vacuole. 
The contents of the vacuoles are very varied. Occasionally solid 
particles may occur, as for instance, those of calcium sulphate in 
certain Desmids. Such particles have been observed by G. S. West 
in a number of green algae (Price, 1914). Apart from these are 
particles of ultra-microscopic dimensions which have been observed 
by Price. Undoubtedly, however, as the facts of osmotic pressure 
and turgor show, the contents of the vacuole consist chiefly of an 
aqueous solution of various substances. Thus in the bulb of the 
onion and in the root of the beet the substance is mainly sugar (De 
Vries, 1884); in the bean, pea, buckwheat and maize it is said to 
be chiefly potassium nitrate (Copeland, 1897); in other plants 
2—5 
