STEM OF EQUISETUM. 179 



bundles, arranged annularly in a central cylinder, as in Gymno- 

 sperms and Dicotyledons. This is the case with Equisetum. 

 E. arvense (the common Horse- 

 tail), can be obtained pretty well 

 everywhere, and will be used 

 therefore for a brief study of its 

 essential structure. A cross - 

 section shows a hollowed stem, 

 the hollow (m) formed lysigen- 

 ously, by the breaking down 

 of tissues, and not infrequently 

 containing water ; a ring of 

 vascular bundles of very re- 

 duced dimensions, each with 



an air-passage (cl) ; surround- 



, , , , Fio. 71. Cross-section through an inter- 



ing tne ring IS an endodermiS node of a barren shoot of Eqiiisetum arvense 



(e) ; a thick cortex with larger <* 11 ^ r - 



air-passages (vl) alternating with the smaller ones of the vascular 

 bundles ; the exterior of the stem showing alternating ridge 

 and furrow, the ridges with strings of sclerenchyma fibres, (hp) 

 between which the chlorophyll - containing cortical tissue (ch) 

 extends to the epidermis, and here stomata are found (st) ; while 

 the median portion of each furrow also shows a sclerenchyma 

 string (hp). The air-passages in the cortex underlie the furrows, 

 and hence are called vallecular passages ; those in the vascular 

 bundle underlie the ridges, and hence are called carinal. 



Directing our attention more closely to the vascular bundles, 

 we see that they are collaterally constructed ; wood towards 

 interior, bast towards exterior (Fig. 72). In the wood the water- 

 containing carinal intercellular passage is very recognisable 

 (cl) ; it is surrounded by thin-walled xylem parenchyma. Into 

 the intercellular passage isolated rings project, derived from the 

 stretched and torn annular tracheides of the protoxylem ; other 

 much stretched vascular tracheides are close by them. The xylem, 

 apart from these vestigial remains, is situated at the sides of the 

 bast, and consists of a number of unstretched resistant vascular 

 tracheides, thickened annularly and spirally. The bast shows no 

 recognisable alternation of broad and narrow elements, but none 

 the less consists here also of sieve- tubes and bast-parenchyma, 

 these latter recognisable by their abundant contents, and perhaps 

 recognisable nucleus. Externally are the narrow protophloe'm 



