136 MORPHOLOGY OF TISSUES. 



above. An exhaustive description of these deviations^ would far exceed our space, 

 and would lead us to great difficulties, since there is at present no portion of the 

 field of botany so unworked as the anatomy of plants properly so called. What 

 follows will only serve to direct the attention of the student to some of the most 

 striking deviations from the processes described under the head of the second type, to 

 which some reference will again be made at the conclusion of the account of the 

 structure of Dicotyledons in Book II. 



(a) The variation is only slight when the increase in thickness is not uniformly con- 

 centric, but is altogether suspended on one side, as in the roots of Polygala Senega, or 

 is much stronger on one side than elsewhere, as in the root of Ononis spinosa and in 

 many stems. 



(b) The cambium is sometimes formed only in the fibro-vascular bundles, which in 

 consequence alone produce secondary xylem and phloem, while the bridging over of the 

 medullary rays by cambium does not take place, and interfascicular secondary fibro- 

 vascular masses are therefore not formed. The fundamental parenchyma which lies 

 between the bundles increases like them slowly in thickness by the extension of its 

 cells and their occasional division, as for example in the stem of Cucurbita, where 

 the bundles are also singular in possessing a layer of phloem on the inside of the 

 xylem, a peculiarity belonging also to Solanaceae and Apocynaceae. But in old stems 

 this ends in an over-bridging of the medullary rays by meristem. 



(c) The processes deviate much more widely from the normal type in those cases 

 where the activity of the original cambium soon ceases, and new bundles are produced 

 out of a meristem in the surrounding cortex ; these bundles first increasing in thickness 

 by cambium, but then ceasing to grow, when a new ring of bundles is formed in a new 

 meristem ; so that at length the wood consists, not of a compact mass, but of concentric 

 layers of isolated fibro-vascular bundles. This process presents a certain resemblance 

 to what takes place in the Aloinese (Type i). The rather frequent instances of this 

 mode may, according to Niigeli and Eichler, be classified into two groups, according as 

 the ring of meristem which produces the new bundles is formed in the primary or 

 secondary phloem. 



Each successive ring of bundles originates in the primary cortex in Gnetum (Gymno- 

 sperms), in Menispermaceae, and in Rhynchosia scandens among Leguminosae. Each of 

 the secondary bundles has on its outer side a border of phloem, which in Gnetum even 

 forms true bast. 



The new rings of fibro-vascular bundles originate successively in the secondary 

 cortex, according to Eichler in Dilleniaceae, many Leguminosae {e.g. Bauhinia, Caulo- 

 tretus), some Polygalaceae {Securidaca and Comesperma), Ampelideae {Cissus according to 

 Criiger), and Phytolaccaceae. 



(d) 'The stem of Malpighiaceae and Bignoniaceae ' (Eichler I.e.) 'at a later period 

 often agrees so far with the foregoing, that the wood appears as if broken up by 

 layers of a parenchyma resembling cortical tissue into a number of isolated portions, 

 of which only the outer ones are merismatic. The history of development is, 

 however, quite different. It does not consist, as in the previous cases, of an actual 

 new formation of vascular bundles outside those already in existence, but in a subse- 

 quent vigorous cell-multiphcation within the medullary rays and the layers of wood- 

 parenchyma which intersect the fibro-vascular bundles. All the portions of wood 

 which are by this means more or less driven apart and displaced are therefore only 

 portions of the primary ring of fibro-vascular bundles ; it is obvious that only the outer 

 ones are capable of increase, since cambium is found in them only.' The wood 

 however does not always break up longitudinally into pieces that are actually separated ; 

 sometimes it is only, as seen on transverse section, divided into lobes, or shows deep 

 indentations, which appear from without as deep furrows, running usually in a spiral 



^ [Oliver has collected the bibliography of the stem in Dicotyledons in the Nat. Hist. Review 

 1862, pp. 298-329, and 1863, pp. 251-258.] 



