iooqI STOREY— ANATOMY OF ISOETES 3 21 



I9O9J 



occur in such a slow-growing stem, and a difference in time would 



run 



claimed 



certain cases, giving as evidence sections which show tracheids at the 

 out>ide and parenchyma in the center. But since the xylem is made 

 up of leaf traces which curve down from the leaves, as in fig. 4, it * 

 evident that it would be possible to obtain a section which sno« s 

 this condition without being an example of protoxylem differentia- 

 tion. In voung stems it is not uncommon to find that at least one 

 section shows this apparent differentiation into protoxylem and metax} - 

 lem. As was mentioned above, an examination of a series of cross- 

 sections of leaf traces as they approach the central axis indicates mat 

 there is no differentiation into protoxylem and metaxylem in that part 

 of the leaf trace. Even if it were present in the leaf traces, tne 

 transverse arrangement of the tracheids would cause it to appear in 

 iers rather than in vertical strands, as in other ptendophyte stem- 

 The cambium, which appears very early (fig. j), begins its ac uwy 

 in the parenchyma which surrounds the central axis, so tna a 

 fcsues which are found outside this thin layer of parencn> ma a 

 secondare The secondary tissues of Isoetes have always! . 

 described as anomalous, and so have furnished a fertile tie 1 

 observation and theorization. The cells which the cambium cuts 



S 



alw 



u, «mi anu uieurizauou. xum w*-~ h have 



externally are ordinary thin-walled parenchyma cells, wnic m 

 avs been called cortex. Whether they represent ancestra p ***» 

 *> of course, an interesting question, but there is nothing ^1 ^ 

 structure to suggest an answer. This is the great storage « -^ 

 th e plant, and the amount of this tissue is much greater 



In the middle and outer region* 



««mea internallv bv the cambium, in u«= »- mesophvll 



<* the cortex the "cells become rounded, often lobed, as in tne - ^ 



(fig 



-»« Wg. 10 ), ana always luiiw... «~ ~ - lthoU(rn there is a 



«Wex increases its thickness from year to year, anno h ^^ 



continual loss by the sloughing-off of the outer layer; 

 re gion there is little or no starch. « j h^ 



The tissue formed internallv to the cambium, the so-cau £ ^ 

 *fc layer/' is that which has aroused the greatest mte ^ 

 ***** cut section this layer is a glistening wfote and sta . 

 h ^lv from the surrounding region. In stained sections 



