BOTANY 



PART I 



their contents. As a rule they remain functional during one vegetative 

 period only. Before passing into the inactive condition their sieve- 

 plates become covered by highly refractive plates of CALLUS (Fig. 65 C\ 

 which diminishes or prevents the exchange of materials between the 

 members of the sieve-tube. If the sieve-tube resumes its function 

 in the succeeding vegetative period this callus is again dissolved. 



The callus plates consist of CALLOSE, a substance the chemical composition of 

 which is still unknown ; this is characterised by its insolubility in ammonia- 

 oxide of copper and its solubility in cold 1 % solution of potash. It is coloured 



reddish-brown by chlor-zinc-iodide, 

 a shining blue with aniline blue, and 

 T, shining red with corallin (rosolic 



acid). 



(b) Vessels. Special, and 

 ultimately dead, cells, which 

 are tube-like with a circular or 

 polygonal cross-section and are 

 elongated and arranged in longi- 

 tudinal rows in the main direc- 

 tion of conduction, serve for the 

 conduction and storage of water 

 in the plant. The lignified walls 

 of these vessels have striking 

 and characteristic thickening. 

 So long as they are functional 



the V6SSels C0ntain Watei> > and 

 often also a limited amount of 

 a i r4 They are distinguished as 



TRArTTFTr> nnf j TT > ArTTW A V 

 \ KAL.HJCA b,. 



The tracheides are single cells 

 with pointed ends, and are as a rule of narrow diameter. Their walls 

 bear peculiar pits (Fig. 70 B). These elements frequently serve as 

 mechanical tissue, as in the stems of Coniferae. The tracheae, on the 

 other hand, are wider or narrower tubes formed from a number of 

 cells by the disappearance of their end walls. When the latter are 

 transversely placed they are completely dissolved, leaving only a 

 narrow annular rim which becomes further thickened (Figs. 67 (7, s, 

 69 /. q, q '). Obliquely placed, end walls, on the other hand, are usually 

 not pierced by a single large opening but by a number of elliptical 

 openings placed one above the other (scalariform perforation, Figs. 

 69 //., 173 tg). Some of the end walls are not perforated but merely 

 pitted, and the vessels are thus of limited length. 



Some tracheae, in particular those of woody climbers or lianes, may be some 

 metres in length. In the Oak also tracheae two metres in length are frequent. 

 As a rule, however, they do not exceed 1 m. and are usually only about 10 cm. in 



FIG. M.-A, Junction of two elements of a sieve- 

 tube of Vltis vinifera, the oblique wall being shown 

 in section, (x 600. After DE BABY.) B, A similar 

 wall in surface view showing the sieve-pits. (Dia- 

 grammatised by ROTHERT after DE BARY.) 



