510 



PHYSIOLOGY. 



\ 



Telacontevtais composed of elongated cylindrical cells, sometimes 

 called hyplice, united end to end into filaments, and either simple or 

 branched laterally, interwoven irregularly into a kind of felted mass. 



This tissue occurs in the 



thallus of Lichens, forming -pig. 558. 



the internal or medullary sub- 

 stance, also in the thallus of 

 some Algae. The mycelium 

 of the Funiri is likewise com- 

 posed of felted cellular fila- 

 ments forming a free cottony 

 mass (fig. 1 B, p. 8). 



Vascular Tissue. Vas- 

 cular tissue is formed by 

 the absorption of a portion 

 of the contiguous walls be- 

 tween cells, so that they 

 tecome converted into con- 

 binuous tubes of more or 

 less considerable length. 



When the constituent 

 cells have spiral-fibrous 



Secondary th tckeningS, they Section of the cells of the seeds of Sophora japonica : 

 TT P T a, thickened cell-walls; o, cavity of the cells 



are Usually Ot prOSeilCny- (bounded by a double line). Magn. 400 diam. 



matous form, and they over- 

 lap each other so that the lines of union are oblique : sometimes 

 these spiroid tubes are distinguished as vessels from those formed 

 of the usually shorter, mostly wider, and more or less flat-ended 

 cells which have pitted walls, and which are called dotted or pitted 

 ducts. 



The dotted ducts are connected with the spiroids through the scalari- 

 form vessels, but in their extreme forms are very unlike, and are found in 

 very different situations. 



The vessels, like the cells, may be spiral, annular, reticulated, 

 or scalariform. They also present special forms hereinafter men- 

 tioned. The constituent cells may be long or short ; in the 

 latter case the vessels are sometimes called moniliform. The spiral- 

 fibrous structure often remains when the primary membrane is ab- 

 sorbed at the surface of junction, so that the constituent cells of a 

 vessel are merely separated by a kind of " grating " of bars. 



Spiral vessels (fig. 559) are found in the youngest and most delicate 

 parts of the plants in which they occur. They are the parts of the woody 

 structure first developed in stems ; they are extensively developed in the 

 ribs of leaf-stalks and leaves, and almost exclusively constitute those of 



