594 SECONDARY CHANGES. 



the Stem of Phytolacca has the structure of perfectly typical soft bast ; in the other 

 investigated cases there is usually a great preponderance of parenchyma, with only 

 very narrow and isolated sieve-tubes. These may therefore be easily overlooked, 

 and are to be further investigated ; where I examined them carefully (Mesembryan- 

 themum, Atriplex sp.) they had the typical structure. In all cases the elements of 

 the phloem are united to a narrowly circumscribed group, and not scattered over 

 a wide area. Sclerenchymatous fibres have not been found in the secondary phloem 

 in any case investigated. 



The intermediate tissue appears in extreme cases in two different forms, namely 

 as thin-walled large-celled parenchyma, or as sclerotic spindle-shaped woody fibres. 

 The former is in all cases sappy, rich in products of assimilation, and often shows 

 an independent growth, which is long continued even at a great distance from the 

 cambium : this growth is connected with cell divisions, and exercises an important 

 influence upon the whole structure. Intermediate forms between the two named, 

 such as thick-walled ' xylem parenchyma,' &c., are not wanting, but require more 

 exact investigation. The latter remark applies also to the distribution of both forms. 

 If only the most fundamental phenomena be taken into account, the following 

 rules may be applied to the occurrence of the two kinds of tissue : — • 



(i) All intermediate tissue consists of delicate, large-celled parenchyma, rich in 

 products of assimilation. Fibrous elements do not occur at all (root of Mirabilis), or 

 only in immediate company with the vessels, that is, as constituents of the strands. 

 This is the case in most fleshy roots of this category, e. g. Beta, and in the stem 

 of Phytolacca dioica. 



(2) The intermediate tissue consists both of parenchyma and of elongated, 

 spindle-shaped, sclerotic, fibrous elements ; this is the case in — 



{a) The extremely hard woods of the shrubs and small trees of the steppes, of 

 the genera Halimus, Caroxylon, Haloxylon, &c., and the tougher stems of other 

 Chenopodiacese, Amarantacese, Nyctaginese, and Mesembryanthema, the chief and 

 fundamental part of which consists of the latter; parenchyma appears to be distributed 

 among it in small groups, like^jhejbundle-parenchyma of normal wood, but this 

 requires more exact investigation. It is^tenTound'in radial bands of various size; 

 but most especially, almost always, in several layers surrounding the phloem portions 

 of the vascular bundles externally and laterally. Where the latter are arranged in 

 concentric zones, it thus forms zones of parenchyma, or consisting mainly of 

 parenchyma. The cells of this parenchyma which accompanies the bundles are 

 sometimes, like those of the ordinary fascicular parenchyma of firm normal woods, 

 very thick-walled, and pitted ; but in most cases the layers immediately surrounding 

 the phloem remain thin-walled and sappy. Like the phloem-strands of Strychnos, 

 they with the groups of phloem fill up spaces in the hard mass of xylem, which are 

 round- or oblong in transverse, section, and which in dry woods are usually for the 

 most part empty by reason of the shrinking of the delicate tissue. I have only 

 rarely found exceptions to this, e. g. in the Salsola investigated. As far as investi- 

 gation extends, all the elements mentioned are arranged radially with considerable 

 regularity in the firmer woods in question. Gronlund states that there are numerous 

 medullary rays in the wood of Neea. 



{b) Those cases are but little worthy of remark, in which both thin-walled 



