SECONDARY THICKENING. NORMAL DICOTYLEDONS. f~^] 493 



canals often arise in them'; in the wood of Prunus avium, they are, according to 

 Wigand ^, a principal starting-point of the disorganisation which produces the cherry- 

 gum. 



The dilatations of the medullary rays 

 and the medullary spots occur, it is true, 

 relatively seldom ; they are, however, 

 characteristic of many woods, both 

 Dicotyledonous and Coniferous. Ac- 

 cording to Kraus and Nordlinger they 

 have been frequently observed in Betula 

 alba, dahurica, populifolia, Crataegus 

 oxyacantha, monogyna, pyracantha, cor- 

 data, Cydonia vulgaris, Pyrus prunifolia, 

 Amygdalus communis, Cotoneaster mi- 

 crophylla, Prunus spinosa, Salix aurita, 

 Caprea, bicolor, Rhus Cotinus, Liihea 

 grandifolia, Pterocarya caucasica, Vac- 

 cinium Myrtillus, Vitex incisa, Calluna 

 vulgaris, Erythroxylon grandifolium, 

 Guazuma ulmifolia, and Liquidambar 

 styraciflua, besides the plants mentioned 

 above ; while they occur rarely in Alnus 

 viridis, Catalpa, Magnolia acuminata, 

 and Salix triandra. I leave unmentioned 

 the cases marked (?) by Nordlinger. 

 Among the Coniferous woods Kraus 

 found them in Abies balsamea, Pindrow, 

 Pichta, Picea orien talis, and Juniperus 

 excelsa ; in Abies pectinata, Cedrus Deo- 

 dara and Larix, he only found swellings 

 of the medullary rays. In Dicotyle- 

 donous woods the passages occur chiefly 

 in the lower part of the stem, and are 

 thence continued into the roots; they 

 also extend, however, into the apices and 

 branches, though here they are less 

 numerous and constant. In the Coni- 

 ferous woods their course has not been 

 minutely investigated ; according to 



Dippel's statements (/. c), which concern this particular point but little, those in the 

 white Fir which contain resin canals may be traced longitudinally for considerable 

 distances. 



Fig. 208. — Hnus silvestris ; radial longitudinal section througfh 

 the wood of a branch, a — e ends of tracheides, with bordered 

 pits (/', ^") in superficial view; clt portion of the young wall of a 

 tracheide with bordered pits still immature ; the further develop- 

 ment of the latter and the contraction of the c.inal in the succes- 

 sion a—c; d, e mature condition ; s, t large pits on the boundary 

 between tracheides and cells of the medullary ray (550). From 

 Sachs' Te.\tbook. 



b. The ligneous bundles. 



Sect. 149. The structure of the ligneous bundle within an annual ring differs 

 according to the presence or absence of the particular forms of tissue, and according 

 to the distribution of those which are present, and the form and structure of each 



* Kraus, I.e. — Dippel, Botan. Zeitg. 1863, p. 253. 

 ^ Pringsheim's Jahrb. III. p. ii8. 



