No. 449-1 STUDIES ON THE PLANT CELL. 371 



regions of the plant, e. g. vascular and certain supporting and 

 tegumentary tissues, are still called cells. When among the 

 lower forms and at certain periods in the life history of many 

 higher plants the protoplasm is naked (e. g. zoospores, sperms, 

 eggs, etc.), these structures are cells in exactly the sense used by 

 zoologists. We shall consider almost entirely the protoplasmic 

 portion of the plant structure for any extended treatment of the 

 walls would lead us at once into that field of microscopic anatomy 

 termed histology. 



i. Protoplasmic Contents. 



The most highly differentiated region of the cell is the nucleus,, 

 a structure remarkably uniform in organization among all plants 

 except the lowest Algae and some very simple Fungi. These 

 more primitive conditions will be considered in Section. VI. 

 Besides the nucleus there are present plastids in all groups 

 except the Fungi. Plastids are likewise specialized protoplasmic 

 elements although much simpler in structure than the nucleus. 

 Nuclei and plastids lie in a protoplasmic matrix called the cyto- 

 plasm. Cytoplasm is more variable in structure and activity 

 than any other region of the cell. Thus three forms of proto- 

 plasm, nucleoplasm, plastidplasm and cytoplasm comprise all the 

 living material of the cell and may be sharply contrasted with the 

 non-protoplasmic contents, mostly food material and waste prod- 

 ucts, which will be considered under a separate head. Definite 

 masses of nucleate protoplasm, with or without plastids are 

 termed protoplasts and such are either unicellular organisms 

 themselves or units of a multicellular structure. 



(a) The Nucleus. 



The nucleus is bounded by a delicate membrane that is 

 probably largely or wholly a modification of the surrounding cyto- 

 plasm. The nucleoplasm very rarely completely fills the nuclear 

 membrane, the remaining space being occupied by a fluid known 

 as the nuclear sap. The elements in the resting nucleus consist 

 chiefly -of material that takes the form of a net work so that the 



