130 



greater thickness of the exine as well as of the intine. Thus the effect of a 

 special layer between exine and intine is produced by the high focal depth 

 of the electron microscope. In reality, this layer is nothing else than an 

 overlapping of the intine and exine, due to oblique cutting. 



The outer part of the exine consists of a thin layer provided with large 

 processes. It shows a high electron density and no inner structure could be 

 resolved so far. The processes are finely papillate and exhibit a considerable 

 variation as to size and shape. Tangential sections through the outer part 

 of the exine (Figs, d-e) reveal the presence of a thin superficial layer (about 

 150-300 A thick) covering the processes as well as the rest of the exine 

 surface. This layer has an extremely high electron density. The inner 

 (infrasporodermal) part of the spore consists of cytoplasm, organellae, and 

 reserve substances. Two kinds of organellae, viz. the mitochondria and the 

 plastids, are easy to identify: the former contain a folded structure, which 

 is also encountered in other plants and in animals; the plastids are consider- 

 ably larger and provided with some traces of lamellae and globular particles. 

 They lack the typical structure found in fully grown and assimilating 

 chloroplasts. In PI. IV, Fig. a, three chloroplasts and part of a fourth can 

 be seen. The membranes surrounding the plastids display an interesting 

 structure. 



In the last-mentioned figure there are many other small bodies with a 

 slightly irregular surface, possibly reserve substances. They appear to 

 consist of two components, a central grain with small electron scattering 

 power (white in the picture), and a surrounding layer of electronoptically 

 dense and homogeneous material. There is reason to believe that these 

 particles constitute a polysaccharide enclosed in a fatty substance. As can 

 be concluded from this description, a tangential section gives a more or 

 less uneven "black" area in the pictures. Organellae and inclusions of other 

 kinds are depicted in Figs, f-i, amongst which are a great number of droplets 

 with a strong electron scattering power. The other three organellae in 

 these figures differ in size and structure. They have a special type of surface 

 membrane which is composed of a thin layer supported by dense droplets, 

 giving, in cross-section, the impression of a pearl necklace. The chemical 

 constitution and function of these organellae are unknown. 



In Paris, in 1954, Frey-Wyssling, when summing up the facts known 

 about the submicroscopic structure of the plant cell, intimated that the 

 fine structure of the ground plasm was influenced by the fixative used but 

 was still largely unknown. He further intimated that the presence of micro- 

 somes and mitochondria had been proved, but that a lamellate structure 

 similar to that demonstrated in the large mitochondria of the kidney had 

 not yet been found. Neither was it possible to ascertain if the nucleus was 

 provided with a true nuclear membrane or not. The last two questions now 



