ANOMALOUS THICKENING IN DICOTYLEDONS AND GFMNOSPERMS. 573 



certain genera it happens that the segments of wood at the sides of the bast-plates 

 grow during the increase in thickness, not only in a radial direction, but also 

 tangentially, and become successively broader, so that they press against the bast- 

 plate, and squeeze it together, displacing and destroying its elements. The same 

 process is repeated in the successive steps, so that old plates are alternately broader, 

 and reduced to quite narrow bands by step-like corners, which encroach simul- 

 taneously upon them, those from opposite sides often coming into contact, and 

 even overlapping one another. Comp. Fig. 226. 



In addition to these diversely varied phenomena there appear finally in the old 

 stem in certain genera, sometimes concentrically renewed rings of growth, sometimes 

 splittings of the first by subsequent intercalary growth ; this will be discussed in 

 later paragraphs. 



The roots of the plants in question have, according to Criiger, in many species the 

 .same lobes, with intruding bast-plates as the stems, with this difference, that the number 

 and arrangement of the lobes and plates are less regular. Bureau's statements contradict 

 this, even in Bign. Unguis, which is speciallyquoted by Criiger, since according to the former 

 bast-plates do not occur, but the thick masses of the xylem composed of vessels and 

 wood fibres are only variously cleft by bands of parenchyma, which sometimes intrude 

 radially, while sometimes they are arranged transversely, and connect the radial bands in 

 a reticulate manner. In the tuberous swellings of the roots, which are characteristic of the 

 genera Glaziovia Bur. and Bignonia Bur., the parenchyma is largely developed between 

 relatively small vascular and fibrous strands. In these cases therefore there is simply the 

 structure of parenchymatous roots. Also in branch-roots two to three years old of An. 

 capreolata I do not find the characteristic stem structure. The xylem is in transverse sec- 

 tion only slightly undulated, composed of sclerotic elements, with medullary rays of one 

 or a few rows of cells ; the bast-layer is thin, with few relatively small sieve-tubes, and 

 small scattered groups of fibres at the outer limit ; the cortex, as noted on p. 547, is per- 

 sistent, and composed of thin-walled parenchyma surrounded by a superficial periderm. 



The generic anatomical characters of the stems of the Bignoniaceae, founded by Bureau, 

 are based partly on the combination of the different phenomena above described, partly 

 on special conditions of structure of wood, cortex, and periderm; and, finally, on the 

 general form of the stem. Since it is of interest to have a complete knowledge of the 

 differences of structure and growth, which obtain within a narrowly circumscribed group 

 of nearly allied plants, corresponding in their mode of life and adaptation, and which are 

 in correlation with the generic characters taken from the reproductive organs, we may 

 here briefly reproduce Bureau's synopsis ', awaiting further explanation of the cases 

 marked *. 



I. Stem always with only four intruding bast-plates. 



A. Bast-plates always equally broad throughout. (Comp. Fig. 224.) 

 (rt) Without subsequent renewed formation of wood in the cortex. 



1. Surface of the cortex with a thin periderm: Arrabidma DC. 



2. Surface of the cortex with a thick layer of cork when old: Paragonia, Bur. 

 {*b) With subsequent renewed unilateral formation of wood in the cortex and 



irregular form of the stem resulting from it : Callichlamys, Miq. 



B. Bast-plates widened in a step-like manner outwards. (Comp. Fig. 225.) 

 a. Stem cylindrical or ridged without channels on the ridges. 



{a) Without stony elements in the cortex. 



a. Steps of the bast-plates broad, i.e. of the width of several segments of 

 wood separated by small medullary rays. 



' Compare Bull. Soc. Bot. de France, /. c. 



