iSOiiTES JAPOKICl, A. BE. 



843 



2. Secondary JPhloem. 



■ 



The cambinm, as stated above, arises very early. It at once begins to cut off new 

 tissues both internally and externally. 



Towards the interior the cambium cuts off a tissue the true morphological nature of 

 which has, in the past, provided the subject for much discussion. This tissue consists 

 of compact angular cells more or less regularly arranged in radial rows, and completely 

 surrounds the primary stele of the stem with the exception of the apical region of the 

 latter, forming a zone which is thickest in the oldest {i. e. lowest) region of the stem 

 (text-figs. 1, 2). 



Considerable interest attaches to this tissue, which is, in many respects, unique in the 

 Vegetable Kingdom. 



Von Mohl (24), to whom we owe the first really scientific description of the structure 

 of the genus Isoetes, suggests that the secondary tissue consists entirely of parenchyma. 



According to Hofmeister (16) a few spiral cells become added to the wood of old 

 vigorous plants as a result of the c^xmbial activity. 



Russow (26), as a result of his anatomical investigations upon herbarium material 

 of several species of Isoetes, arrived at the conclusion that the tissue cut off internally 

 by the cambium consists of parenchyma interspersed with tracheids and wdth cells 



which he referred to the phloem. 



Hegelmaier (15) refers to this tissue in I. velata and in /. Diirieui, but leaves its 

 morphological nature an open question. He w^as apparently handicapped by the poor 



microscopical techniq^ue of his day. 



Farmer (14) gives a detailed description of the various elements comprising this tissue 

 in I. lacustris, but leaves the question of their exact morphological nature undecided. 



Wilson Smith (30) considers the adoption of the term ' phloem ' inadvisable until 

 further evidence as to the physiological properties of the cells in question is available. 

 He remarks [I. c. p. 227) that " no clearly defined sieve-tubes, the essential elements of 

 the phloem, have been found either in the stem or in the leaf." Also, " the inner cells 

 of the prismatic zone are unknown to become secondarily thickened and transformed 



into xylem tracheids." 



Scott and Hill (28), whose investigations were confined to L Rijstrix, k tevrcstrial 



species, confirmed Russow's original suggestion that this tissue was made up of both 

 xylem and phloem together with a certain amount of parenchymatous tissue. 



The most recent description of the secondary tissue of Isoetes is given by Miss Stokey 

 (32), who was fortunate in having material of four diff'erent species at her disposal. 

 She maintains {I. c. p. 333) that '' the so-called * prismatic layer ' is secondary xylem. 

 The cambium does not form phloem." Moreover, she adds that there is no primary 

 phloem in the stem. This conclusion is of special interest, since the four species 

 examined exhibited a very considerable range in habit. That undoubted phloem occurs 

 both in the leaf and in the root, is admitted; if this be so, it is diflicult to under- 

 stand how the products of metabolism can be conveyed from one part of the plant 



SECOND SEKIES. — BOTANY, VOL. VIII. 



3d 



