84 



BOTANY 



such as the Papaveraceae, of which the Poppy (Papaver) or Celandine 

 (Chelidonium), with its characteristic orange-coloured " sap," are familiar 

 examples, or the Compositae, of which in particular the Lettuce (Laduca) 

 may be cited. Latex vessels are distinguished from latex cells only 

 by the method of their development, which has resulted from the 

 fusion of rows of elongated cells, the separating transverse walls of which 

 have become more or less completely absorbed. In this manner a net- 

 work of latex vessels, such as that in the Spanish Salsify (Scarzonera 

 hispanica), may be formed (Fig. 91). In the formation of the sieve- 

 vessels, or sieve-tubes as they are usually 

 termed (Fig. 92), the cell fusion is much 

 less complete. It is confined to fine canals, 

 which perforate the cross-walls, which are 

 known as sieve-plates. The cells thus 

 united by the sieve-pores remain as distinct 

 segments of the sieve-vessels. It is worthy 

 of special note that, despite the fact that 

 after the nuclei of the different sieve-tube 

 segments become disintegrated, their cyto- 

 plasm still continues living. The walls 

 of sieve-tubes are always unlignified. 

 Their sap cavities contain a watery, and 

 more or less diluted, solution of albumin- 

 ous substances, which, by means of the 

 pores of the sieve : plates, may pass from 

 one segment of the sieve-tube to another. 

 As a rule, small starch grains may also 

 be found in the sieve-tubes. The pores 

 of the sieve-plates never attain great 



Pig. 91.— Tangential section through , . . _. r & 



the periphery of the stem of dimensions (H lg. 92, B, D), and are gener- 

 scorzonem Mspanica, showing re- ally extremely small. Sieve-plates are 



tieuiately united latex vessels. sometimes found algQ in ^ ^^ ^^ 



of the sieve-tubes, and the sieve-pits per- 

 mit communication between adjoining sieve-tubes. Such lateral sieve- 

 plates are frequent in the Conifers (Figs. 85, 93). As a rule, sieve- 

 tubes only remain functional for a short time. After their activity 

 ceases, the sieve-plates become invested with the strongly refractive 

 callus-plates (Figs. 92 C, 93 B) already referred to (p. 80) in dis- 

 cussing callose. 



A cell fusion also takes place in the formation of vessels or 

 tracheae, but it should not be considered as a union between living 

 cell bodies, but merely as one between cell cavities. The vessels are 

 formed by the absorption of the transverse walls of rows of cells, 

 the lateral walls of which are peculiarly marked by spiral or reticulate 

 thickenings, or, as is more frequently the case, by bordered pits. In 

 cases where the transverse walls are at right angles to the side walls 



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