CELLS AND TISSUES 5 



The principal types of plastids are chloroplasfs found in photo- 

 synthetic tissues (chlorenchyma), and colorless leucoplasts which 

 occur in meristematic tissues and in the cells of storage organs. 

 The latter are sometimes more specialized, and are known as 

 amyloplasts because of their relation to the formation of starch 

 reserves. Chromoplasts, other than chloroplasts, may contain 

 many of the color pigments found in flowers and fruits. The caro- 

 tene in the fruit of the tomato occurs inside the plastids; and, after 

 their decomposition, in free crystalline or globular form. The 

 yellow and a few of the red pigments of many flowers and vegetative 

 structures may be localized in plastids; while others are in solution 

 in the cell sap, such as the anthocyanins in the fleshy root and 

 hypocotyl of beet or radish. 



Various cell inclusions may be sufficiently characteristic to 

 be helpful in anatomical diagnosis. These include crystals of cal- 

 cium oxalate, calcium carbonate, silica, inulin and protein crystals, 

 aleurone grains, starch, and semi-solid compounds classed as gum, 

 resin, fat, mucilage, or latex. The cell sap may contain salts, 

 carbohydrates, proteins, alkaloids, and pigments in solution which 

 can be used in micro-chemical diagnosis, and similar techniques 

 are applied in cell wall analysis. 



The Cell Wall. — With few exceptions, the cells of seed plants 

 have walls that are formed by the activity of the protoplast. The 

 character of the cell wall is important in tissue analysis since 

 various cell types have been named on the basis of differences in 

 its structure. The mature wall is generally regarded as a non-living 

 membrane; but it is commonly penetrated by cytoplasmic strands, 

 plasmodesma, which may be either primary or secondary in origin. 

 The primary type is present during the formation of the primary 

 wall and persists thereafter, while the secondary type develops 

 later by penetration. The plasmodesma may be aggregated into 

 groups to form conspicuous strands, or sometimes occur as evenly 

 distributed single threads — in which case they are difficult to 

 demonstrate because of their extreme fineness. The penetration of 

 the wall by these cytoplasmic threads is thought to establish proto- 

 plasmic continuity in living tissues, and it is probably not inac- 

 curate to regard the young wall as consisting in part of living 

 matter. In any event, the interrelation between the development 

 of the wall and the protoplast is very intimate, since the growth of 

 the former is conditioned upon the activity of the latter. 



