FORMATION OF CELLS. 



15 



projects inwards at the indentations in the form of acute-angled ridges. The division 

 of the simultaneously contracting and rounding masses of protoplasm, proceeding from 

 without inwards, now becomes more evident if they are freed by pressure (b' f'^ or by 

 solution of the cell- wall in sulphuric acid ; they then have the appearance of four-lobed 

 bodies. The separation at length proceeds so far that the four segments part from one 

 another ; but since the formation of the cell-wall proceeds simultaneously, each of these 

 now lies in a chamber surrounded by a cell-wall (c). Later each protoplasmic body 

 (young pollen-grain) forms a new wall around itself, the thick common cell-wall is 

 dissolved, and thus the four pollen-grains become free. 



2nd Case. When there is no perceptible contraction of the dividing protoplasm ^ ; the 

 cavity of the mother-cell remains completely filled by the daughter-cells ; these therefore 

 are not rounded off, the derivative cells appearing as segments of the mother-cell. 



As in the preceding case, so here also 

 we must distinguish, according as the cell- 

 wall is formed only after the division, or ad- 

 vances during the division from without in- 

 wards. In both cases the newly formed 

 cell-wall gites the impression of a lamella 

 thrust in between the derivative cells, 

 which becomes joined to the wall of the 

 mother-cell ; it is usually called a division- 

 wall ; its direction and position are of 

 great importance in morphology ; it always 

 stands at right angles to the connecting 

 line of the centres of the new cells. In 

 this mode of cell-division, there is, with 

 rare exceptions, a bipartition of the mother- 

 cell 2; it is the invariable process in the 

 formation of tissues, but occurs also dis- 

 tinctly, though less conspicuously, in the 

 formation of spores and pollen. 



a. The protoplasm arranges itself in 

 the interior of the mother-cell in two por- 

 tions, the boundary-surface of which is 

 already visible before the partition-wall of 

 cellulose is formed ; this partition-^all arises 

 simultaneously at all points of the boundary- 

 surface as a thin membrane ; it is only when 

 it afterwards increases in thickness. that it 

 sometimes splits into two lamellae, one of 

 which belongs to each of the sister-cells ^. 



This mode of Cell-formation may be very clearly observed in the formation of the 

 pollen of some Monocotyledons. Fig. 12 shows the process in Funkia o'vata. In /the 



Fig. 12.— Mode of formation of the pollen of Ftmkia ova 

 ( X 550). In VII the wall of the daughter-cell has absorbed wate 

 till it burst ; its protoplasmic body is forcing itself through the 

 cleft, and is lying before it, assuming a spherical form. 



^ Hofmeister (Handbuch der physiol. Botanik, I. p. 86) supposes in this case also a contraction 

 of the contents, in order to get room for the division-wall ; since, at the time the molecules out of 

 which it is formed are separated out of the protoplasm itself, a change in position takes place, by 

 which the particles of protoplasm approach one another a little ; though it is doubtful whether this 

 is the necessary result of the plan indicated in Figs. 8 and 11. 



2 In many hairs {e.g. Trad^cantia) a division into more than two daughter-cells placed in a 

 row occurs simultaneously. (A. Weiss, ' Die Pflanzenhaare ' in Karsten's Botan. Untersuchungen, 

 p. 494.) 



^ It should be remarked at the outset that in tissue-cells the division-wall of two cells is a 

 lamella common to both, the growth and interior differentiation of which usually proceeds equally 

 on both sides. (Compare sect. 4 under b, and Formation of Tissue.) 



