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THE AMERICAN NATURALIST [VouXLII 



The subject of polar organization in plant cells has received 

 especial attention recently through the work of Harper 1 on the 

 mildews (particularly Phyllactinia), and that of his student 

 Marquette- on Isoetes and Mars'dia. Harper found in the ascus 

 of Phyllactinia an especially favorable subject for the study of 

 the organization of the resting nucleus and its mitosis in rela- 

 tion to a central body, which is a permanent organ of the cell and 

 gives to the nucleus and cell a definite polar organization. The 

 central body lies on the nuclear membrane and the chromatin 

 elements in an early stage of mitosis have the form of 

 strands, each one attached independently to this center. Even 

 in the resting nucleus, when the chromatin has the structure of a 

 network, its attachment to the central body is evident and indi- 

 cates that the strands have an individual connection with the 

 central body as a nuclear pole. During the prophase of mitosis 

 the central body divides and the two portions move apart to 

 become the poles of spindle and, at the completion of mitosis, 

 each remains connected with a daughter nucleus, thus main- 

 taining its polarity. Furthermore, in the process of nuclear 

 fusion within the ascus each nucleus contributes its complete 

 polar organization so that the resulting fusion nucleus has for 

 a time two central bodies and two independent groups of chro- 

 matic elements, which later gradually fuse into a single system 

 with one central body. 



Marquette has discovered a remarkable polarity in the leaf 

 cells of Isoetes. Each resting cell contains a large starch-con- 

 taining body, which lies so closely pressed against the side of 

 the nucleus that the latter is usually indented. Sometimes the 

 body is present without the starch grains that usually render it 

 very conspicuous. Previous to nuclear division this polar struc- 

 ture elongates and divides by constriction and the two parts 

 draw apart so as to form a furrow on the surface of the nucleus. 

 The separated halves then come to lie at opposite ends of the 

 nucleus, which has become somewhat elongated. These develop- 

 ments take place before the appearance of the chromatin gives 



^larner. R. A. Sexual Renrnilne.t.inn find the Organization of the 



