46 THE CELL-WALL. 



brane, destined to constitute a partition-wall, for open communications such as the 

 above. For segments of the wall of various sizes are made from the beginning with 

 sieve-like perforations, as is shown in fig. 10 4 , which represents diagrammatically 

 portions of tubular cells called " sieve-tubes." The pores are crowded close together 

 on the perforated areas of the walls of the sieve-tubes, and their dimensions are 

 relatively broad and short. Thus, when two neighbouring protoplasts reach out to 

 one another through these pores, that is to say, when there is continuity of the 

 protoplasm of the two cell-cavities, the connecting filaments, which pass through the 

 pores and which fill them completely, are short and thick and have the appearance 

 of pegs or stoppers. 



But in many cases the pores through which adjoining cell-cavities communicate 

 are drawn out to a great length, forming infinitesimally slender passages. They are 

 situated close together in great numbers and penetrate transversely through the 

 thick cell- walls (fig. 10 5 ). Neighbouring protoplasts may be brought equally well 

 into mutual connection by means of these canals, or perhaps it would be better to 

 say that their connection may be equally well maintained. For it is very probably 

 the case that in the first rudimentary partition- wall, which is produced between the 

 products of division of a protoplast, minute spots remain open and are occupied by 

 connecting threads common to both halves of the protoplasm as they draw apart. 

 Then in proportion as the partition-wall between the two protoplasts, produced by 

 the division, becomes thicker, the openings take the form of fine canals, and the con- 

 necting filaments are modified into long and exceedingly fine threads which fill the 

 canals. These protoplasmic threads pierce through the thickened cell-wall in the 

 same way as a dozen telegraph-wires might be drawn through a partition from one 

 room into another. Often a number of protoplasts living side by side and one 

 above the other are linked together by filaments of this kind, which radiate in all 

 directions. 



This species of connection, of which an intelligible idea is given by fig. 10 5 , 

 escaped the notice of observers in former times owing to the extraordinary minute- 

 ness of the canals, and delicacy of the protoplasmic filaments. Another method of 

 communication between protoplasts in adjoining cells has, on the other hand, been 

 long known and often described, its phenomena being very striking and visible when 

 only slightly magnified. The connection referred to is that which is afforded by 

 the formation of so-called "vessels." By vessels the older botanists understood 

 tubes or utricles, arising from the dissolution of the partition-walls between a series 

 of cells. Either the partition-walls in a rectilineal row of cells vanish, in which 

 case long straight tubes are produced; or portions of the walls of cells arranged at 

 different angles to one another are dissolved, and then tubes are formed having an 

 irregular course, and sometimes branching or even uniting, so as to make a net- work. 

 In instances of the first kind the lateral walls of the series of cells which are to lose 

 their transverse partitions are previously thickened and made stiff by the proto- 

 plasts, which also provide them with various mouldings and panellings, and above all 

 with bordered pits. This task accomplished, the protoplasts forsake the tubes, whose 



