THICKENING. 



131 



drawn from the side-w r alls of the sieve-tube. The "slime-string," 

 therefore, is due to the action of the alcohol. While these con- 

 ditions hold good for the Cucurbitaceae, active sieve-tubes show 

 usually a non-coagulable cell-sap. In the lining layer of cyto- 

 plasm are found larger or smaller masses of slime, and small 

 colourless leucoplasts, which usually produce very small starch 

 grains rich in amylo-dextrin (see p. 124), and colouring red with 

 iodine. In material examined in a fresh state we again find the 

 contents of the tube collected unilaterally at the sieve-plate, 

 and the starch grains accumulated there. 



Thickening of Dicotyledonous Stems. An extraordinarily 

 favourable object for the study of the growth in thickness of 

 Dicotyledons is the pipewort, Aristolochia Sipho, a hardy deci- 

 duous climbing plant not infrequently grown in England, and 

 material for the investigation of which will therefore be probably 

 not difficult to obtain. It is desirable to collect it at the end of 

 June, using it then, or preserving it in alcohol. Our description 

 will be made from fresh material, but, with the exception of 

 the green colour, will serve 

 also for alcohol material. 

 First take a cross-section 

 through a twig about 1 or 

 i inch in thickness. This 

 section (Fig. 48) shows with 

 a lens an internal loose pith 

 (m), around this a ring of 

 separate vascular bundles 

 (fv), farther outside a con- 

 tinuous white ring (sk), then 

 green cortical tissue (c) (if 

 alcohol material the green FlG . 43. -Cross-section through a twigoMm- 



colour will be absent), and Mochia Sipho, about i inch in thickness, m, 

 n 11 11 -i pith ; fv. vascular bundles, of which vl is the 



finally a yellowish - green wood ; and ^ the bast portion ; fc, fascicular 



With cam bium ; ifc, inter-fascicular cambium ; p, pri- 

 mary bast parenchyma on the outer side of the 

 the bast, which ett'ects a transition to the ground- 

 tissue ; pc, pericycle ; sk, sclerenchyma ring, 

 belonging to the pericycle ; e, starch sheath 

 mine that the pith consists (endodermis) ; c, green cortex ; cl, collenchyma 



( x 9\ 



of large, round cells, in part 



filled with air. In the vascular bundle the wood (vl) appears 

 darker, pierced by the large cavities of the vessels ; then 

 follows the cambial band (fc), composed of narrow, radially- 



peripheral rind (cl). 

 a low power under 

 microscope we can deter- 



