COMBINATIONS OF CELLS. VESSELS. 



513 



Fig. 563. 



or flat ends, where" they adhere together; they vary in diameter, like the 

 spiroids, and sometimes present on their walls large pits or spaces, covered 

 with a kind of line network of fibres, as in the clathrate cells described 

 in a former section. 



Ceils of this character (which differ from the conducting-cells before 

 alluded to, in that the latter are destitute of markings or pits) always occur 

 in the middle of the fibre-vascular bundles of Monocotyledons ; and they are 

 intermixed, mostly in alternate layers, with the liber in the fibrous layer 

 of the bark of Dicotyledons. They are strikingly distinguished from spi- 

 roids by containing thick and opake sap, while the latter usually contain 

 only air when fully developed. 



Casparv includes under the head of conducting-cells not only those 

 cylindrical tubes before alluded to, but also elongated cells having the form 

 and appearance of vessels, but which do not form continuous tubes, being 

 separated one from the other by partitions formed by the adjacent ends of 

 the cells. 



Laticiferous vessels, or milk-vessels, containing the latex or milky 

 juice of such plants as Poppies, Euphorbias, Cichoraceee, &c., 

 are formed from series of cells, the parti- 

 tions between which become very early and 

 speedily obliterated. The constituent cells 

 may be placed one over the other to form 

 ultimately a straight tube, or, more generally, 

 the lateral partition-walls between the cells 

 become obliterated, and the result is a branch- 

 ing tube, or series of tubes, which, according 

 to Trecul, anastomose with other kinds of 

 vessels, and allow the contents of the one to 

 pass into the cavity of the other. They occur 

 most abundantly in the pith and inner layers 

 of the bark, in roots, leaf-stalks, &c., often 

 forming a complete network. Dippel says 

 the laticiferous vessels replace the clathrate 

 or latticed vessels of other plants. Their 

 presence is most easily demonstrated by boil- 

 ing a fragment of tissue in weak solution of 

 potash. 



Vesicular vessels (Hanstein) resemble simple 

 unbranched laticiferous vessels, containing a 

 milky juice. They are formed of rows of cells 

 Disposed lengthwise, and their partition-walls 

 are thickened and perforated as in the sieve Lati^iferous canals from the root 

 cells (see p. 487). This form of vessel occurs in of Dandellon ' Maga ' 10 diam ' 

 the bulbs of Onions and other Monocotyledons. 



In Oommelynaceue and in Pandanaceae long rows of cells are met with 

 filled with raphides, and ultimately forming continuous tubes, which are 

 stated by Hanstein to be homologous with latex-tubes. 



Tyloses. In some instances, as in the Vine, vesicular formations may 

 be seen in the interior of the large vessels. According to Von Mohi, 

 they are produced by a protrusion of the adjacent cell, which penetrates 



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