330 



CONDUCTING SYSTEM 



finally traversed by closely crowded equidistant circular or polygonal 

 pores. 



In a certain sense, therefore, the sieve-tubes of Gymnosperms are 

 comparable to tracheides, while those of Angiosperms correspond to 

 genuine vessels. The width of the smallest known sieve-pores hardly 

 exceeds the transverse diameter of the exceedingly minute pores that 

 traversed the limiting membranes of ordinary pits. At the opposite end 

 of the scale must be placed the sieve-pores of such plants as Cucurbita 



or Lagenaria, which may reach a 

 diameter of o/u. ; this is another 

 instance of the prevalence of large 

 conducting channels in climbing 

 plants. 



Sooner or later every sieve- 

 plate becomesinvested bya peculiar 

 highly refractive substance,to which 

 Hanstein has given the name of 

 callus. These callus-pads consist of 

 a special carbohydrate,c//ose,which 

 is characterised by the fact that it 

 is insoluble in cuprammonia, but 

 soluble in a cold 1 per cent, solu- 

 tion of caustic potash and in soda 

 solution. It is further stained 

 reddish-brown by chlor-zinc-iodine 

 The callus does not merely cover , 

 both surfaces of the sieve-plate, but also lines the walls of the sieve- 

 pores (Fig. 136, ca) ; the pores consequently become partially or 

 entirely blocked. Since, however, callosc is a readily soluble sub- 

 stance, the contracted or occluded pore may be reopened at a later 

 stage. In many cases, therefore, as was noted long ago by Wilhelm, 

 callus serves the purpose of regulating the width of the sieve- 

 pores in accordance with variations in the intensity of translocation. 

 The formation of callus at the beginning of winter, for example, often 

 leads to the complete occlusion of the sieve-pores, and hence to an 

 entire cessation of the flow of material in the sieve-tubes. In the 

 spring the sieve-tubes may be reopened by the dissolution of the 

 callus-pads. Such an alternate opening and closing of the tubes takes 

 place, for example, in Vitis and in certain other woody Dicotyledons, as 

 well as in many Monocotyledonous rhizomes. In the majority of plants, 

 however, the deposition of callus results in the permanent closure of the 

 sieve-pores, and for this reason only takes place in old sieve-tubes 

 which are about to become inactive. 



Fig. 133. 



Small portion of a leptome-strand of Layenaria 

 vulgaris, in T.S., showing two sieve-plates, m and n, 

 each occupying the whole extent of a transverse 

 septum of a sieve-tube. One sieve-plate (to) has 

 wider meshes than the other (n). x375. After De 

 Bary. 



and bright blue by aniline blue. 



