Tllh: STRiJCTUUAL COMPONENTS OF PROTOPLASTS 33 



The arrangement is such that between each two protein layers there are 

 two films of chlorophyll molecules, a double layer of lipide molecules, a 

 few xanthophyll molecules, and water. The distance between two of the 

 protein layers is about 0.005/u. To what extent this represents the true 

 structure no one can say at present, but hypotheses, right or wrong, are 

 valuable as long as they are subject to experimental test. 



In some algae the chloroplast contains one or more peculiar bodies 

 known as pyrenoids (Fig. 21). These appear like small masses of protein 

 in the midst of the stroma and evidently play some definite role in the 

 elaboration or deposition of carbohydrates, for starch grains develop in 

 their immediate vicinity and commonly form a dense mass about each 

 of them. Such behavior strongly suggests localized enzymatic activity. 

 In the liverwort Anthoceros and the pteridophyte Selaginella there are 

 compound pyrenoids, the parts of which give the appearance of trans- 

 formation into starch granules. There are no known pyrenoids in seed 

 plants. 



Development and Multiplication of Plastids. — When the meristematic 

 tissues in a young, actively growing bud of a seed plant are examined, it 

 is found that the plastids there do not have their mature characters, but 

 appear rather as very small globules or rodlets; these are plastid pri- 

 mordia, or proplastids. As the young stem tip and leaves grow and the 

 cells multiply, the proplastids also grow and multiply by division. This 

 is apparently their only mode of multiplication, for there is as yet no 

 conclusive evidence that they ever arise anew in the cytoplasm. Sooner 

 or later, when the leaf tissues become further differentiated, the enlarged 

 proplastids become transformed into chloroplasts. Division may occur 

 even after they are fully differentiated. In root meristems the story is 

 essentially the same, except that leucoplasts rather than chloroplasts are 

 formed. 



The division of the young plastids during the development of most 

 tissues is not definitely correlated in time with that of the nuclei and cells. 

 However in some plants, such as the liverwort Anthoceros and various 

 algae having only one plastid per cell, the plastid divides regularly just 

 before or during the division of the other cell elements, thus preserving 

 the one-plastid condition. It should be added that many plastids 

 undergo striking alterations in shape and that these should not be 

 interpreted too hastil}^ as stages of division. 



The bud tissues described above are derived from embryonic cells and 

 ultimately from the fertilized egg. This raises the question of the con- 

 tinuity of plastids as individuals, multipl^dng only by division, through 

 successive life cycles. In the angiosperms, proplastids appear to be 

 present at all stages in the formation of spores and gametes, the plastids 

 thus being continuous from one generation to the next. The inheritance 



