ELECTRON MK.HOSCOPY 



Fig. 11. Section through a proplastid from Be- 

 gonia, X25,000. (By K. Muhlethaler) 



Fig. 12. Section through a further developed 

 proplastid from Begonia, X 25,000. {By K. Muhle- 

 thaler) 



Fig. 13. Section through a chloroplast of Elo- 

 dea canadensis, X 16,000. (By K. Muhlethaler) 



Stem. This shows more or less the same struc- 

 lurc as the mitochondria. Next (2) the size 

 iiicroasos and the plastid center, containing 

 a tul)ular structure, is formed. Starch grains 

 now appear (3), and then the material for 

 the formation of the layer structure pro- 

 trudes radially from the center (4). The 

 starch breaks down (5) and the lamellae 

 l)egin to form. They multiply by thickening 

 and splitting until the continuous lamellar 

 structure of the chloroplast (G) is formed by 

 the fusion of short lengths. The resulting 

 plastid is traversed by continuous double 

 lamellae (7). Finally further splitting forms 

 the grana of the fully differentiated chloro- 

 plast (8). 



It is interesting to compare Figure 10 

 with the electron micrographs of Miih- 

 lethaler showing plastid development in 

 Begonia and the chloroplast of Elodea 

 canadensis (Figures 11-13). The proplastid 

 from Begonia (Figure 11) is generally similar 

 to the drawing of the barley proplastid 

 (Figure 10) and the further development of 

 the Begonia plastid (Figure 12) is like stage 

 (4) of the description. The fully-differ- 

 entiated chloroplast of Elodea canadensis 

 (Figure 13) is also generally similar to that 

 of barley. From this it may be inferred that 

 the general pattern of chloroplast develop- 

 ment is similar in different species. 



It is not possible to study the structure of 

 the chloroplast in detail here. The structure 

 is generally similar in all plants, but there 

 is much variation in the dimensions and 

 spacings of the lamellae. The chloroplast is 

 the site of photosynthesis, the chlorophyll 

 being concentrated in the grana. The grana 

 are distributed in the stroma which forms 

 the body of the chloroplast. 



The Nucleus. Sections through the nu- 

 cleus show chromosomes in the despiralized 

 condition and with good resolution a fine 

 granular structure can be detected in the 

 chromosomes and nuclear cytoplasm. How- 

 ever, the electron microscope has added 

 little to our knowledge of chromosomes. 



I 



88 



