210 Hichols, A morphological study of Juniperus communis var. depressa. 



protoplasm of the mother cell into four parts takes place, and the 

 cytoplasm which surrounds each of the nuclei encloses itself with 

 a thin cell membrane, but that no thick walls, like those described 

 by Miss Ferguson, are formed. From the following account, how- 

 ever, it will be seen that the conditions in var. depressa are 

 remarkably similar to those in Firnis. That these features have 

 not been more frequently observed may be accounted for by the 

 fact already pointed out by Miss Ferguson that the thickened 

 walls usually react very poorly to stains. 



The structure of the microspore mother cell at the completion 

 of the homotypic division is shown by fig. 35. The four nuclei of 

 the tetrad are connected with one another in all directions by 

 kinoplasmic fibers. Some of these represent connecting fibers which 

 were in the spindle of the homotypic division, and which have not 

 yet disappeared. Others, however, arise de novo from the cyto- 

 plasm, and to all appearances these are exactly like the true spindle 

 fibers. The fibers soon become thickened at the equatorial planes, 

 giving rise to the six cell plates which separate the four nuclei 

 (fig. 36). Another important change, however, has already taken 

 place in the mother cell. During the anaphase of the homotypic 

 division there begins to appear in the peripheral region a trans- 

 parent, homogeneous layer which stains bluish with gentian violet 

 (figs. 33 — 35). This becomes thicker, and when seen in section 

 appears as a broad band entirely surrounding the protoplasmic 

 contents of the cell. That this layer represents merely the swollen 

 inner wall of the mother cell seems doubtful, for it appears very 

 intimately related to the enclosed protoplasm, which by this time 

 has shrunk away from the mother wall. It seems rather to be 

 an entirely new wall developed in anticipation of the formation 

 of microspores. The cell plates now split, and walls are laid down 

 in the usual manner (fig. 37). These walls then apparently swell, 

 become continuous with the thick enveloping wall (fig. 38) and 

 assume the same reaction toward stains. In this manner the cells 

 of the tetrad become separated from one another and from the 

 outside by a thick, transparent wall, while the original mother cell 

 membrane, already very thin and distorted, gradually disorganizes. 

 The cells of the tetrad, while still enclosed by this thickened layer, 

 begin to form the walls which are present in the mature micro- 

 spore. These are sometimes evident in places where an enclosed 

 cell has become separated from the thickened wall. The pollen 

 grains are eventually liberated by the dissolution or breaking down 

 of the enveloping wall. 



There can be little doubt that the peculiar structure described 

 here, and found in Pimis, is in the nature of a cell wall and is 

 not merely aviscid or liquid substance, and an examination ofliving 

 material of J. communis fully confirms the observations made on 

 fixed and stained preparations. Yet while in Pinus it "is left 

 behind as a definitely outlined wall after the escape of the spores" 

 and "the empty mother-cell with its four Chambers is often met 

 with", in Juniperus it is not of such a permanent character 



