78 



HUTCHINSON'S POPULAR BOTANY 



FIG. IOS.-PAKEN- 

 CHYMA FROM THE 



*Equi- 



etum arvense). 



In certain cells of the Ferns and their allies the thicken- 

 ing deposit laid down on the inner surface of the cell- 

 walls takes the form of miniature ladders, on which 

 account the vessels constructed out of these cells (though 

 absorption of transverse septa is rare in Ferns) are called 

 scalar if or m, or ladder-like (Lat. scala, a ladder). As a 

 matter of fact, scalariform vessels are only modifications 

 of the reticulated form, from which they differ by the 

 partition-walls of secondary deposit being larger and more 

 regular. 



Spiral and annular vessels occur in the stems of most 

 Dicotyledons (plants with two seed-leaves), but only in 

 what is known as the primary wood, which forms the 

 first circle round the pith, and is called on that account 

 the medullary sheath (Lat. medulla, the marrow of bones) ; whereas reticu- 

 lated and pitted vessels are found in the denser internal parts of the 

 woody layers (vide Chapter VII.). All of these occur in the leaf-stalks and 

 veins of leaves, and in certain parts of the flower, but never in the bark. 

 They keep the cellular tissue of the leaves stretched and extended, acting 

 like the ribs of an umbrella. In Monocotyledons (plants with only one 

 seed-leaf), they are placed in the interior of the woody bundles of the stem, 

 and sometimes you will meet with them in the root-fibres. In the mature 

 state they contain nothing but air ; but occasionally, in the spring, a portion 

 of the sap sucked up by the roots is pressed into them a process on which 

 depends, for example, the " weeping " of wounded grape-vines (Thome'-s 

 Lehrbuch, p. 30). 



There is one other kind of elongated cell found in the 

 woody parts (fibro-vascular bundles) of many plants which 

 should not be passed over. "We have described it as " woody 

 fibre," but the scientific name for these vessels is bast-tubes 

 or bast-fibres (fig. 105, 6). Bast-tubes must not be confounded 

 with what are known as sieve-tubes or bast-vessels. The 

 former are long, pointed, and thick-walled, and occasionally, 

 though very seldom, they are branched. The sieve-tubes 

 or bast-vessels, on the other hand, consist of slender flexible 

 tubes, with their walls unmarked by secondary deposit 

 (fig. 105, s). The dividing walls of the cells of which 

 the last-named vessels are built are not entirely absorbed, 

 as are the partition-walls in the bast-fibres ; but they are 

 perforated in various places so as to resemble a sieve, 

 whence they are called sieve-plates, and the vessels, as we 

 have seen, sieve-tubes. Not infrequently the side walls of 

 adjoining tubes are also perforated. 

 vitalba). If two or three hollow cylinders, covered at each end 



FIG. 109. PEOS- 

 ENCHYMA OF 



