2 PROTOPLAS:\I AND PLANT CELLS 



shape (mostly rounded) and separated from the cyto- 

 pkism 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- 

 sion the centrosome appears as a minute 

 piaim enclosed by body lYi closc proximity to the nucleus. It 



a cell wall. ,i ,• ■• i ,... 



takes an active part m 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 



