2 PROTOPLASM AND PLANT CELLS 
shape (mostly rounded) and separated from the cyto- 
plasm by a delicate membrane. Like the cytoplasm, the 
bulk of the nucleus seems to be a colorless fluid in which 
is found a network of fine threads (the linin network) 
on which occur more or less numerous coarser or finer 
granules of chromatin. A rounded, usually nearly homo- 
geneous body, the nucleolus, is mostly visible as a small, 
highly refractive drop within the nucleus. (3) Centro- 
some. Although of general occurrence 
throughout the animal kingdom centro- 
somes are definitely known only in certain 
of the lower plants. In a cell not in divi- 
—-"-J sion the centrosome appears as a minute 
plaag cidicsaiky body in close proximity to the nucleus. It 
or ee takes an active part in nuclear division in 
animals, and possibly may do so in those plants in 
which it is present. (4) Plastids. These consist of 
denser masses of protoplasm lying in the cytoplasm 
and are colorless (leucoplasts) or colored (chloroplasts 
and chromoplasts). They are lacking in the cells of 
many plants. 
3. All these forms of protoplasm possess many char- 
acteristics in common, both as to physical and chemical 
structure. They are very complex compounds with 
most of the characteristics belonging to the proteins but 
differing from them in some important points. Proto- 
plasms consist mainly of carbon, hydrogen, oxygen, nitro- 
gen and sulphur and of phosphorus also in the case of 
the nucleus. In all probability certain metallic elements 
also enter into the combination. 
4. The most remarkable property of protoplasm and 
that which distinguishes it from all other chemical sub- 
stances is its power of manufacturing new protoplasm 
out of simpler substances, in other words, the power of 
