STRUCTURE OF VESSELS AND TRACHE1DES 309 



fibres ranges from one to four or even more. The individual fibres are 

 often forked in places ; or successive turns of the spiral may be con- 

 nected by oblique anastomoses. The first-formed vessels, which are 

 differentiated while the young organ is still growing in length, are 

 always annular or spiral, because it is only these two types of thicken- 

 ing that admit in any considerable degree of a subsequent longitudinal 

 extension of the thickened walls. The thickening rings or spirals do 

 in fact move further and further apart in these young vessels, as the 

 organ grows in length. The reticulate and the pitted types of thickening 

 are connected by insensible gradations. 



A special feature of the thickening fibres has recently been brought 

 to light by Eothert, 15S who has demonstrated that in most plants these 

 fibres are more or less distinctly 

 contracted immediately above their 

 insertion upon the unthickened mem- 

 brane (Fig. 127). The contracted 

 basal portion is often sharply de- 

 limited from the broader part, so that 

 the whole fibre is |-shaped in 

 cross-section. In other cases the 

 two regions pass insensibly into 

 one another. The cross-sectional out- 

 line of thickening fibres is altogether 

 very variable. Eothert believes the 

 physiological value of the narrow base 

 to consist in the circumstance, that this mode of insertion of the 

 thickening fibre causes the smallest possible loss of permeable surface 

 in the shape of unthickened membrane. Schwendener had long pre- 

 viously attributed a similar significance to the peculiar structure 

 of bordered pits (see below). Eothert himself indeed institutes a 

 comparison between these pits and the annular, spiral and reticulate 

 thickenings. Clearly, however, the increase in the area of unthickened 

 and hence readily permeable wall-surface, produced by the narrow 

 insertion of the thickening fibres, is inconsiderable, unless the fibres 

 are very closely crowded. There are, moreover, quite a number of 

 plants (Equisctum, Cactaceae, etc.) in which the insertion of the fibres 

 is not contracted at all. 



All the above-mentioned modes of thickening (the bordered pits 

 excepted) are designed to serve the same end, namely, to give sufficient 

 rigidity to the conducting tubes without unduly interfering with diffu- 

 sion between adjacent elements. The necessity for the provision of 

 special strengthening arrangements arises from the circumstance that 

 tracheides and vessels, being dead structures and hence unable to 



Fig. 127. 



L.S. through two spiral vessels of Cucurbita 

 Pcpo. After Rothert. 



