74 



HUTCHINSON'S POPULAR BOTANY 



In a few forms of vegetable life the protoplasm of two or more cells 

 coalesces for the purpose of reproduction, and this is known as conjugation. 

 Here (fig. 102) are some cells of a little fresh-water weed, Zygnema 

 quinium, common enough in our ponds and ditches, and popularly known 

 as Silkweed or Crow-silk. Each of the pale yellow-green filaments represents 

 a separate plant, and is built up of a single row of cells ; but when conjuga- 

 tion is about to commence, the cell-walls of two distinct filaments that 

 happen to float in proximity form blunt projections from their sides, and 

 reach out to one another till they meet. Then, at the points of contact, 

 those portions of the walls which hinder communication between contiguous 

 cells dissolve away ; the sap at once occupies the passage thus formed ; and 

 the protoplasm from one of each pair of united cells, forcing its way through 

 the narrow channel, fuses with the protoplasm in the companion cell, and so 

 conjugation is effected. 



But a far more common method of increase than any which we have yet 

 considered is that which is known as cell division. Increase in length of 



FIG. 103. INDIRECT NUCLEAR DIVISION (vide text). 



every filamentary plant of Silkweed was due to cell division ; the cells of 

 the fragment of Onion-skin which we were speaking of in the previous 

 chapter multiplied in this way ; so did the star-shaped cells of the Common 

 Bean lately mentioned. Indeed, the vegetative organs of most plants (as 

 distinguished from the reproductive organs) are almost always so formed. 



But what is cell division? To say that all normal vegetable growth 

 takes place by such means is no explanation of the process ; we are only 

 moving in a circle. Will you follow an attempt to illustrate the process 

 by means of a few diagrams ? We will suppose that the first sketch (fig. 102) 

 represents a row or part of a row of vegetative cells, of which the upper- 

 most is about to divide. Here (fig. 103) is this cell on a larger scale, with 

 its cell-wall (6) and its granular protoplasmic contents (c), in the midst of 

 which is drawn a circular disc to represent the nucleus (ri). Changes in the 

 nucleus intimate that the process has commenced. The nucleus elongates. 

 and its delicate fibrillce delicate even under the highest powers of the 

 microscope appear at this stage to interlace in a confusing manner. A 

 little later the tanglement is over, and the fibrillce are seen to be converging 

 to one or the other of the poles of the nucleus. Between these fibrillce 



