ELEMENTARY STRUCTURE. 



35 



one or more turns of a spiral, the two forms irregularly alter- 

 nating with each other {fig. 68). These vessels are of rarer 

 occurrence than the other modifications of spiral vessels. In 

 size they vary from about 4^ to ^^^^ of an inch in diameter. 

 Annular vessels occur especially in the vascular bundles of 

 the stems of soft rapidly growing herbaceous plants among 

 Dicotyledons ; also in those of Monocotyledons ; and in Flower- 

 less Plants. In the latter they exist especially, and. of a very 

 regular character in the Equisetacea. 



Fig. 66. Fig. 67. Fig. 68. Fig. 69. Fig. 70. Fig. 7]. 



Figs. 66 and. 67. Annular vessels .Fig. 68. Vessel 



showing a combination of rings and spiral fibre. j-^--— , 



Fig. 69. Reticulated vessel Fig. 70. Prismatic ' / S^d^\ r^ 



scalariform vessels of a Fern Fig. 71. Cylin- ' Sl^^l^ f^^- 



drical scalariform vessels of the Vine. ^^ — 



d. Reticulated Vessels. — In these vessels the spiral convolu- 

 tions are more or less irregular, and connected in various ways 

 by cross or oblique fibres, so as to form a branched or netted 

 appearance {fig. 69). These vessels are generally larger than 

 the annular, and of much more frequent occurrence. They are 

 found in similar situations. 



e. Scalariform Vessels. — These are but slight modifications of 

 the reticulated vessels, and owe their peculiarity to the sides 

 being marked by transverse bars or lines, aiTanged over one- 

 another like the steps of a ladder, whence tlieir name {figs. 70 

 and 71). It is frequently difficult to determine whether this 

 appearance of lines or bars is caused by the secondary deposits or 

 fibres, or whether it is not owing to the presence of spots which are 

 thinner than the surrounding parts of the wall, as with the pits of 

 pitted vessels. Probably in most instances at least, this is 

 true; while in others, it would appear to be caused by the fibre, as 

 it sometimes happens that scalariform vessels may be unrolled, 

 as is the case with the true spiral vessels. Scalariform vessels 

 thus appear to form an intermediate stage between pitted and 

 spiral structures. These vessels are sometimes cylindrical tubes 

 like the other kinds, as in the Vine {fig. 71) and many other 

 Dicotyledonous Plants, in which condition thev are apparently 



d2 



