96 SIEVE-TUBE ELEMENTS 



slime-plugs are artifacts rather than structures peculiar to nor- 

 mal uninjured sieve-tube elements. The slime in sieve-tube ele- 

 ments originates from the disintegration of the slime-drops which 

 are commonly present in angiosperms as inclusions in the cyto- 

 plasm of 3'oung cells ; the disintegrated nucleus and the sieve-tube 

 sap also become part of the slimy contents of the sieve tubes. In 

 many plants, Icucoplasts and starch grains may be observed in the 

 sieve-tube elements (cf. Esau, 1939, pp. 384-386). 



2. Sieve-plates and sieve-fields. In addition to the ultimate 

 loss of the nucleus, the mature sieve-tube element is characterized 

 by the presence of sieve-plates which occur in various regions of 

 the cell wall. The origin of the term "sieve-tube" rests upon 

 the erroneous idea that, in such a plant as Cucurhita, the end- 

 walls of the elements are perforated like a sieve. But modern 

 studies agree in showing that sieve-plates are not literally open, 

 perforated areas in the wall. On the contrary, the so-called pores 

 in the sieve-plate appear to be penetrated by either delicate or 

 rather coarse plasmodesmata. Crafts (1939a) has concluded that 

 these cytoplasmic strands, which thus connect the protoplasts of 

 adjacent sieve-tube elements, are solid rather than tubular in 

 structure as was maintained by certain earlier workers. Con- 

 siderable variation occurs with reference to the distribution of 

 sieve-plates in sieve-tube elements. In highly specialized sieve- 

 tubes (e.g., in Cucurhita), the transverse end-wall is occupied by 

 a single large plate with rather coarse plasmodesmata while the 

 lateral walls are provided with smaller, less distinct plate-like 

 areas. But several sieve areas forming a "compound sieve-plate" 

 may be present in end-walls which are inclined or sloping. In 

 the gymnosperms, very numerous small sieve-i)lates or "sieve- 

 pits" occur on the radial lateral walls (Abbe and Crafts, 1939). 

 The term ^'sieve-field" was originally applied by Niigeli to the 

 apparently reduced sieve-plates present on the lateral walls of 

 angiospermons sieve-tube elements. However, as Esau (1939, 

 p. 395) has clearly indicated, sieve-plates and sieve-fields may 

 intergrade in structui-e so that oidy an arbitrary distinction can 

 be made between them. The morphologiad nature of the sieve- 

 plate is still, to some extent, an unsolved problem. This is largely 

 the case because of the many gaps in our knowledge as to the 



