28 VEGETABLE ELEMENTS AND TISSUES. 



of the free passage of the sap through them. Their 

 walls are invariably covered with markings, consisting 

 of either simple or bordered dots, resembling those 

 met with in the preceding forms of tissue. The ducts 

 are often easily recognizable with the naked eye, 

 in transverse sections of stems, by the large pores 

 which they form in the wood. These may be well 

 seen in a section of a piece of cane. The tissue com- 

 posed of dotted ducts is called bothren'chyma (ftoOpos, 

 pit) ; but the term is principally applied to those 

 ducts in which the dots are simple, i. e. have no inner 

 dot. 



The structure of the above forms of tissue may be 

 best understood in relation to their development. It 

 has been stated that the essential part of the cell is 

 the protoplasm. As cells grow older, new matter is 

 deposited by the protoplasm upon the inner surface 

 of the cell-wall, either to a small extent, evenly and 

 uniformly, as in ordinary parenchyma, or unevenly, in 

 the form of spiral layers, forming fibres or bands, 

 leaving bare spaces, where the original cell-wall exists 

 alone. The matter thus deposited is called secondary 

 deposit, the original cell- wall being the primary de- 

 posit. When the secondary deposit covers the in- 

 terior of the cells except at certain slit-like spaces, we 

 have the appearance figured in PL I. fig. 6 a). When 

 the deposit forms a spiral fibre, or a series of rings, 

 we have the spiral or annular vessel or duct. And 

 when the interspaces between the coils of a close 

 spiral fibre are filled up except at certain spots, we 

 have the dotted or reticulated vessel or duct. 



In many instances, these deposits are present to- 

 gether : thus, sometimes the outermost deposit leaves 

 rounded pits or dots, while an inner portion forms a 

 spiral fibre (PL I. fig. 11 b) or one layer leaves simple 

 rounded pits, while the other leaves smaller slits or 

 dots placed opposite the former (PL I. fig. 6 b). 



In some cells the cavity is almost entirely filled up 



