1. The root tip and cambium of
Pinus Strobus were fixed in several different fluids so as to preserve the chromatin, plastin, and mitochondria in the following combinations:-(1) chromatin, plastin, and mitochondria fixed; (2) plastin and mitochondria fixed and chromatin dissolved; (3) chromatin and plastin fixed, mitochondria dissolved; (4) plastin fixed, mitochondria dissolved, chromatin rendered unstainable; (5) chromatin fixed, mitochondria dissolved, plastin ren-dered unstainable. It was thus possible to separate the plastin from the chromatin, chemically, and to follow it through the several mitotic phases unobscured by any other nuclear constituent. It was also possible to distinguish any possible granules of
plastin in the cytoplasm from the mitochondria.
2. The organization of nucleolar material in both the resting and dividing nucleus of
Pinus differs in many important details from that recently described in a number of angiosperms. The resting nucleus of Pinus contains an average of six nucleoli which are in intimate contact with the chromatin threads of the reticulum. On the initiation of karyokinesis, each nucleolus becomes attached to the spireme. The plastin passes into the spireme and is distributed to the daughter cells as a part of the chromosomes. On the reorganization of the daughter nuclei, the plastin is consolidated into the typical nucleoli of the resting cell. Unlike
Zea, the
Pinus nucleoli do not persist to metaphase, and diserete globules of plastin do not, pass from the equatorial plane to the poles of the spindle. No evidence was found of any fragments of nucleolar material passing out into the cytoplasm.
3. Two distinct substances can be observed in the resting nucleoli in living cells. As these substances have a different index of refrac-tion, the nucleoli appear vacuolate. These substances can be separated by fixation in the resting nucleus and in the later telophases. During the other mitotic phases, however, they have not been separated, so that, at present, it is not possible to trace each one individually through the entire cycle of cell division. Neither substance is chromatin.
4. The bearing of the activity of plastin during mitosis upon several theories concerning the forces involved in cell division is dis-cussed.
The author wishes to express his obligation to Professor I. W. BAILEY for his assistance throughout the course of this investigation and especially for his help in preparing the cambium for cytological investigation. Professor BAILEY's preparation of living cambial tissue made it possible to check the several fixation images wich the struc-tures of untreated cells. The author also wishes to thank Miss ANNA F. FAULL for drawings in Plate 6 and the Text-Figure.
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