swell, their edges become soft and gluey, there- 

 by possibly leading to the development of ab- 

 normal forms. 



Hruby* (1935) after irradiating the spores of 

 Polypodium aureum S with various doses, estab- 

 lished that even the strongest doses did not 

 inhibit their germination. A few irregularities 

 were observed in connection with the three 

 highest doses. Then he repeated his experiment 

 using still higher doses. The experiments indi- 

 cated that abnormalities in the plastids increase 

 proportionately to the dose. For example, with 

 a dose of 2355 r the number of cells with abnor- 

 mal plastids was 21. 6%, while with 6800 r the 

 percentage of such cells went up to 46. 5. Vacu- 

 oles appeared in all the plastids, even in the 

 smallest, before the nuclei of the cells showed 

 any changes whatsoever. To a certain extent 

 Hrub5^ confirms Knudson's observations concern- 

 ing the fusion of small plastids into large, shape- 

 less forms. In the majority of cases, however, 

 the merger of the plastids is explained by the 

 fact that plastids had begun to divide and that 

 this division had not reached completion. 



Later Knudson [1940] published a long article 

 describing the action of X rays on fern chloro- 

 plasts. He selected the fern because it produces 

 spores that are minute in size and simple in 

 structure. Every effect induced by X rays would 

 be direct instead of secondary, as is possible 

 when seeds are irradiated. The irradiation was 

 performed with a Coolidge tube at a distance of 

 12. 5 centimeters, at 78 to 80 kilovolts and 4 

 milliamperes, without a filter. The irradiation 

 was performed intermittently. In one experi- 

 ment the spores were exposed to the following 

 dosages: 7,000, 10,000, 12,000, 15, 000 and 

 20, 000 r. In another experiment dosages from 

 20, 000 to 50, 000 r were administered by incre- 

 ments of 5000 r. Fifty-one prothallia were 

 isolated. 



The classification presented below is based on 

 cells of medium age. In very young cells the 

 plastids are small and undeveloped; in old cells 

 they are likely to change shape due to age. Knud- 

 son divides the visible changes into eleven types: 



Type A— Normal Plastids 



In prothallia from unirradiated spores and 

 nearly all the irradiated ones, the plastids 

 are characteristically uniform in size and 

 shape. They are very small, plate-like or 

 discoid in shape, and have a thickness of 1. 5^- 

 Their area ranges between 3 x 4.5 ^ and 6x9m. 

 Another characteristic of normal cells is the 



^ Editor's note: while the article by Hruby cannot 

 be checked in the original, it is probable from the 

 text of Knudson's 1940 paper and a discussion later 

 this chapter (page 64) that the organism is Equisetum 

 arvense rather than Polypodium aureum . 



large number of plastids per cell. 



Type B— Grouped Plastids 



The plastids are massed about the nucleus in 

 groups. Prothallia with this type of plastids are 

 characterized by thin walls, blue-green coloring, 

 and slow growth. 



Type C— Giant and Amoeboid Plastids 



Plastids of this type of various sizes and 

 shapes assume strange outlines. They are 

 formed by the union of smaller plastids. Later 

 small plastids arise from them by budding. 

 When these plastids have a more or less regular 

 shape, they can be measured. Squarish plastids 

 of this type were 50 x 70 jU. Occasionally, ring- 

 like forms occur. 



Type D— Large, Thin Plates of Irregular Shape 



In cells of medium age, plastids fuse into 

 thin threads. The starch granules in them are 

 large. 



Type E— Very Large, Thin Plates 



Only one prothallium of this type was ob- 

 served. It was a vigorous grower. 



Type F— Irregular, Thin Plates 



In cells of medium age they are linked by 

 thin green strands. In some of the older cells 

 the plastids fuse. 



Type G— Thin Plates of Various Sizes, with 

 Vacuoles 



Type H— Large Plastids 



This type occurs most commonly. Plastids 

 of this type resemble normal ones but are at 

 least twice as large. 



Type I— Thin, Small Plates of Various Sizes 



Some of them are connected by thin green 

 strands, others are vacuolated. The surface 

 of these plastids is irregular due to large starch 

 granules. 



Type J -Small Plastids, Variable in Shape, with 

 a Tendency toward Congregating in Groups and 

 Vacuolating 



Nearly every cell shows a colorless, ring- 

 shaped structure of variable size. 



Type K— Vacuolated Plastids 



This is an unusual type of plastid. In young 

 cells the plastids are completely normal, ex- 

 cept that their contents appear more fluid and 

 more finely grained. In slightly older cells the 



64 



