MINUTE ANATOMY OF FLO WISHING PLANTS 221 



Lalex tubes (/). 



— TSCHIRCH. 



(Fig. 372). The cell fusions may take place mainly in 

 longitudinal directions, giving the 

 semblance of jointed tubes, or in all 

 directions, producing a dense net- 

 work. In the Milkweeds and the 

 Euphorbias the milky juice {latex} 

 is held in elongated, branching, 

 tubular sacs originating as single 

 cells in the emln'yo, and growing 

 with the growth of the plant until 

 they have pushed their way into 

 every part of the plant body. The 

 latex itself is a mixture of a con- 

 siderable variety of substances; 

 sometimes some of the ingredients 

 are poisonous, as, for example, mor- 

 phia, the active principle of opium, found in the latex 

 of the Poppy. 



513. Tissues. — The word tissue has been frequently 

 used al)ove without exact definition, ^-et probably without 

 misapprehension. Technicall}' the term tissue means a 

 mass or collection of cells of the same kind. Any num- 

 ber of cells of a certain kind constitute a particular kind 

 of tissue. Thus collench3'ma, a jjarticular kind of tissue, 

 was descril)ed al)Ove. 



514. Fibrovascular bundles are so called from the fact 

 tliat tliey are made up largely of fibrous cells and vessels 

 (ducts). In a translucent herbaceous stem like that of 

 the Balsam, the bundles may be seen without dissection, 

 as strands hini;' not far beneath the surface, traversino- the 

 entire length of the stem, and giving off branches to the 

 leaves. In the cross section of such a stem these bund'ss 

 wouhl be seen as several — [)erlia.ps five — areas more 

 opaque than the surrounding parenchyma, arranged ap- 

 proximately in a circle (compare Fig. 376). Upon exami- 

 nation with a proper power of the microscope each bundle 

 would Ije seen to consist of three parts (Fig. -373). The 

 inner of these consists largely of wood fibers and ducts. 



