Cleland • Cytology: the Study of the Cell 



235 



Wvssling, Stcinmann, and Sjostrand 

 have revealed an amazing coniplcxitv 

 of organization in bodies which had 

 previoush' been considered to be essen- 

 tiahv structureless. Chloroplasts, which 

 can often be seen under the ordinary 

 microscope to be filled with granules 

 or "grana" are now found to present a 

 finely layered appearance, the grana 

 as well as the surrounding material 

 having such a structure. The grana, 

 which contain the chlorophyll, seem to 

 be composed of alternate la\ers of pro- 

 tein and fatty material and, since the 

 tadpole-shaped chlorophyll molecule 

 is attracted at its head end by water and 

 at the tail end by oil, it takes up a posi- 

 tion with its head in the protein layer 

 and its tail in the lipid layer. Thus the 

 chlorophyll becomes oriented in a very 

 orderly and precise manner. 



Chondriosomes also appear to have 

 a very intricate and precise structure. 

 The work of Sjostrand and others 

 shows that they have a double mem- 

 brane with cross membranes extending 

 part or all the way across the body. 

 Like the chloroplasts, therefore, they 

 also have a form of lamellate struc- 

 ture. 



A third center of interest among 

 cytologists arises from the fact that 

 eytological studies can throw consider- 

 able light on the relationships of 

 species and races, and can furnish clues 

 as to the paths along which evolu- 

 tionary progress has been made. The 

 nature of the evidence which cytology 

 is able to present varies with the ma- 

 terial. In some cases, a comparative 

 study of chromosome structure will 

 indicate relationship. Chromosomes 

 have definitive shapes and sizes, and 

 it is often possible to recognize particu- 

 lar chromosomes under the micro- 

 scope. In many genera, it is possible to 

 compare the various species or sub- 

 species from the standpoint of chromo- 

 some structure. Species which have 

 similar or identical chromosomes so far 



as morpholog)' is concerned arc con- 

 sidered to be more closely related than 

 species whose chromosomes differ in 

 these regards. 



In other cases, relationships can be 

 determined or confirmed by analyzing 

 the structural alterations which have 

 occurred in the evolution of the group. 

 Thus, in certain species of Drnsophila, 

 the fruit-fl\', inversions of chromosome 

 segments have occurred with relative 

 frequency. In some cases, sequences of 

 inversion can be followed, especially 

 where a portion of a previously inverted 

 segment becomes involved in a second 

 inversion. In other organisms, other 

 types of structural alteration have 

 proved of value. For instance, in the 

 evening primrose (Oenothera), the au- 

 thor and his students have found that 

 exchanges of segments between non- 

 corresponding chromosomes ha\e oc- 

 curred with unusual frequency and it 

 has been found possible to analyze 

 many races from the standpoint of the 

 interchanges which have occurred. 

 Races which show evidence, at least in 

 part, of the same interchanges are con- 

 sidered, other things being equal, to 

 have been derived from common an- 

 cestors in which these interchanges 

 occurred. The more closely races resem- 

 ble one another in respect to the inter- 

 changes which have occurred in the 

 course of their evolution, the more 

 closely related they are considered to 

 be, and vice versa. As a result of eyto- 

 logical and genetical techniques, tlrc 

 evolutionary' story of the evening prim- 

 rose is being revealed, and this in a 

 genus which has for many years been 

 the despair of taxonomists who have 

 tried to ferret out the relationships by 

 strictly taxonomic methods. 



In plants another eytological phe- 

 nomenon has proved to be of great im- 

 portance in determining relationships. 

 It not infrequently occurs in plants that 

 the chromosome number becomes al- 

 tered. Chromosomes may occasionally 



