TYPICAL CELLS. 57 



of these cells are so long in proportion to their breadth, and of 

 such diminished calibre, that the} 1 have naturally been called 

 fibres, although all gradations between them and typical cells 

 ma}' be demonstrated. All these cells are interchangeably 

 called woody fibres or wood-cells, and one kind of them takes 

 the name of bast-cells. 



193. Others are of larger calibre, are peculiarly marked by 

 thickenings on certain lines or in certain patterns, incline to be 

 developed end to end in a chain or row, and to become confluent 

 at the junctions, so as to form conduits of considerable length ; 

 these are called vessels, or ducts. Vessels and fibres are 

 associated in the plant ; almost every separate thread of frame- 

 work consists of both, and so is called a fibro-vascular bundle or 

 fascicle. Moreover, the known gradations between the two are 

 such as to render a complete distinction between them nearly im- 

 practicable ; so that they form the fibro-vascular, or, when a 

 single word is used, the vascular system. To this system, also, 

 pertain specially differentiated cells, such as cribrose-cells, in the 

 bark, etc. 



194. All these are developed in or among the fundamental 

 or untransformed cells, and originate from the differentiation of 

 some of them. 



195. The fundamental or typical cells may therefore be said 

 to constitute the fundamental system ; which may also be con- 

 veniently called the cellular system, in contradistinction to the 

 vascular. 



196. In an ordinary leaf it forms all but the framework of 

 ribs and veins ; in the stem of a dicotyledon, the outer bark, the 

 pith, and the rays which traverse the wood ; in that of a mono- 

 cotyledon, which generally has a looser texture than the last, it 

 is the common mass through which the definite bundles of the 

 vascular system are distributed. Of the fundamental system, 

 the most typical or unmodified cells are such as the chlorophyll- 

 bearing cells of leaves and of the green bark of stems, as well as 

 those with uncolored contents forming the pith, etc. Borrowing 

 a word from the old anatomists, the early investigators of vege- 

 table structure called tissues composed of such cells Parenchy- 

 ma, perhaps taking the idea of the name from leaves in which 

 the veins are distributed through the softer parts as blood-vessels 

 through the parenchyma of the glands. 



197. Parenchyma, therefore, is the name of cellular tissue 

 in contradistinction to fibro-vascular tissue. In its primary 

 sense, only comparatively soft and thin-walled cellular tissue 



