10 A TEXTBOOK OF THEORETICAL BOTANY 



Embedded in the cytoplasm are the following structures : — 



{a) The Plastids, which are protoplasmic bodies, denser than the 

 cytoplasm and not separated from it by any definite membrane. 

 They comprise : — 



Chloroplasts, the bearers of the green pigment, Chlorophyll. 

 Chromoplasts, with colours other than green (the term is 



sometimes used to include all coloured plastids). 

 Leucoplasts, which are colourless and found chiefly in 

 underground organs and in the meristem cells. 



(b) The Vacuoles, sac-like enclosures in the cytoplasm, filled with a 



liquid " cell-sap." 

 (f) The Mitochondria or Chondriosomes, granules, rods or threads 

 apparently composed of phospholipins and proteins, which are 

 found scattered through the cytoplasm of all cells. 



(d) The Centrosomes, granules associated with nuclear division, 



which are characteristic of animals, but are found only in a few 

 of the lower plants. 



(e) The Ergastic Substances, or materials secreted by the cytoplasm 



either as food materials or as by-products. Here are included 

 such things as oil-drops, protein grains or crystalloids, starch 

 grains (in the plastids) and crystals of Calcium oxalate. 



3. The Nucleus, almost always single in the cell and composed of the 

 following parts : — 



(a) The Nuclear Membrane, separating it from the cytoplasm. 

 {b) The Chromatin, which is organised as : — ■ 



The Nuclear Reticulum, formed of fine threads, which is 



characteristic of the non-dividing or " metabolic " nucleus. 

 The Chromosomes, which are relatively thick rods, formed 

 from the nuclear reticulum during nuclear division and con- 

 stant both in number and form. Chromatin is a compound 

 of nucleic acid with basic protein. 



(c) The Nuclear Sap, or Karyolymph, which is colourless and fills all 



the central parts of the nucleus. It may also contain ergastic 

 reserves, e.g., protein crystals. 

 {d) The Nucleolus, one or more in each nucleus, which is a spherical 

 granule of material, attached to the reticulum and associated 

 with certain chromosomes. It consists of a mixture of protein 

 and lipin, and normally stains difi^^erently from the chromatin. 



A cell in a higher organism cannot be looked upon as an independent 

 unit. There is a considerable degree of physiological unity pervading all 

 tissues and the cell must be largely controlled by the functioning of the 

 tissues as a whole. Although itself a synthesis of many smaller components, 

 as we have seen above, it plays a part in the synthesis of a still higher unit, 

 the organism, to which it is subordinate. Modern studies no longer treat the 



