VESSELS WITH BORDERED PITS. 1 37 



tissue of the youngest organs, these vessels are gradually constructed in the order 

 here mentioned, and in general their breadth increases in the same sequence ; while 

 the length of the individual segments, or cells, of which the vessels are composed, 

 decreases at the same time and in the same order. Hence the first formed, narrow, 

 annular and spiral vessels, which have to take part in the whole growth in length of 

 the organ, are in the completed state very long tubes ; whereas in the last formed^ 

 pitted vessels, the individual segments, especially when they only attain completion 

 after the termination of the growth in length of the organs, appear short and barrel- 

 shaped. These also may occasionally however, as in roots, form very long tubes. 



The nature of the sculpture on the walls of the vessels has already been 

 explained (in lecture VI). Here is still to be added, that the vessels with 

 bordered pits owe their striking aspect on the surface view to the circumstance 

 that the pits are here not only very closely crowded, and are thus separated 

 fundamentally only by thickening ridges; 

 but the thickening ridges become arched, 

 growing forth from the primary thin cell- 

 wall into the interior of the cell, laterally 

 and on all sides over the spaces between 

 the meshes, so that each pit is connected 

 with the inner cavity of the cell only by 

 means of a very narrow pore or slit. We 

 may also say, a bordered pit is a short 

 canal, leading out from the interior of the 

 cell, and suddenly becoming widened at the 

 primary cell-wall. In the surface view of fig. us.-Pa« of the longitudinal «aii of a pitted vessel. 



,,,.,,, ^ ,., a the primary thickening ridges, which become arched over 



a bordered pit, therefore, a roundish pore the areoloe (/.), and only leave the sllt W exposed. 



or slit is perceived in the middle, which is 



surrounded by a circular or polygonal border, corresponding to the outer cir- 

 cumference of the pit. In the Ferns (as Fig. 144 shows), where the segments of 

 the pitted vessels are situated upon one another with very oblique septa, the bordered 

 pits are so extended in breadth, that the thickening ridges between them often 

 appear like the rounds of a ladder ; hence the old name, ladder-like (scalariform) 

 vessels. In addition to these proper vessels, there are found in the xylem of 

 thicker strands other more fibrous elements, with the upper and lower ends 

 obliquely cut off, or drawn out to long points, the finer structure of the walls and 

 pits of which is similar to that of the true vessels. These, therefore, have been 

 termed tracheides, in contradistinction to the true vessels, which are named 

 tracheae. All these tracheal elements lose their protoplasm, together with the 

 cell nucleus, completely, as soon as their wall-structure is fully developed; so that 

 later not the smallest remnants of them are to be perceived. Even the cell-sap 

 disappears completely, and the vascular tubes only contain air; and indeed it is not 

 improbable that they occasionally become even empty of air. 



The resemblance of the vascular constituents of the phloem, the sieve-tubes, 

 to the tracheal elements of the xylem is very slight. The latter, with their ligni- 

 fied cell-walls devoid of contents, are conspicuous on account of the sculpture of 

 their wall; while the sieve-tubes, on the contrary, are provided with soft, supple, 



