CHAPTER XVI. 



ANOMALOUS THICKENING IN DICOTYLEDONS 



AND GYMNOSPERMS. 



Sect. i8o. The secondary growth in thickness of stem and root differs in a 

 number of Dicotyledons and Gymnosperms from that which is called normal because 

 of its occurrence in the large majority of these plants ; it is therefore called anomalous 

 in the cases in question. 



In the first place, we may range among anomalous forms those not uncommon 

 cases, in which by reason of strongly eccentric growth of the xylem, while the growth 

 in thickness of the surrounding cortex is almost uniform, the stem or root attain 

 forms, which differ greatly from the ordinary conical or cylindrical form ; but in 

 which the other relations are normal. An exquisite example of this series is seen in 

 the upright stem of Heritiera Fomes, described by Schacht \ which, as far as may be 

 judged from the description, grows at first in a manner in other respects normal, and 

 uniformly all round, but later grows in thickness more especially at two opposite 

 corners, so that it attains the form of a board, e.g. 1 1 ft. broad, and only i in. 

 thick. Such phenomena appear in very striking form in climbing stems : species of 

 Cissus, and Piper with strap-shaped stems'^; Cassia quinquangulata'^ with five or more 

 marked prominences, as seen in transverse section, each of these being opposite to 

 one of the orthostichies of leaves ; Lantana spec. * with four longitudinal prominences 

 separated by deep furrows, and alternating regularly with the leaf-insertions, &c. 

 In the above-named climbers there often appears a splitting of the whole stem at the 

 furrows when it grows old. Again, allied but less regular inequalities occur not 

 unfrequently in roots, and are characteristic, e. g. in those of Ononis spinosa 

 described by Wigand ^ All these appearances, considered anatomically, are nothing 

 more than extreme instances of the widespread phenomenon of unequal eccentric 

 development of the woody layers, which appear as specific peculiarities in the 

 examples cited, while in other plants they may appear as phenomena peculiar to the 

 individual, or induced by definite physiological causes. They may therefore be 

 excluded from the subject now under consideration, reference being made to Sects. 

 138 and 140. 



Sect. 181. A special anatomical treatment is, however, demanded by those 



• Lehrb. I. p. 344. ^ Criiger, Botan. Zeitg. 1S50, p. 121. 



'•' Ibid. 1851, p. 469. * Fr. Miiller, Botan. Zeitg. 1866. 



5 Flora, 1856, p. 673. 



