.^2i 



SECONDARy CHANGES. 



arrangement of the secondary elements often continues to be maintained here, or 

 is at least recognisable ; hence, in every radial row derived from the cambium, one 

 or more sieve-tubes always alternate more or less regularly with parenchyma. 



This arrangement appears with quite diagrammatic regularity in the bast of the 

 CupressineK and many Taxineae^. The transverse section of the bast (Fig. 211) 

 here shows regular rows, both in the radial and tangential direction. Every fourth 

 tangential row consists of fibres ; of the three which lie between two fibrous rows, 

 the middle one is parenchymatous, while the outer and inner each form an inter- 

 rupted layer of sieve-tubes. The parenchymatous cells are approximately similar in 

 width to the sieve-tubes (Junipcrus communis), or are wider (e.g. Thuja occidentalis). 

 In the stem of species of Pinus (P. Strobus, nigricans, silvestris), and also of Abies 

 pectinata, irregular tangential rows of wide parenchymatous cells alternate with 



.^,^r n0^^ yR=^^ ^"^^ 



/ 



3 



?c^< 



J 





FIG. 212.— Tilia argentea. Transverse section through the inner bast, cambium, and autumnal boundary of the 

 wood of a branch seven years old (cut in November) (220). The wood is drawn without tlie details of structure of the 

 membranes; A external limit of the autumnal wood, which is sharply limited by the dark outlines of its tangentially 

 flattened elements ; c — c cambium and zone of young secondary growth ; M—m small medullary rays ; in the one to the 

 right are three fibres iy); k crystal-containing sacs, with crystals, some of which were broken by the razor. In the 

 bast-strand, between the two rays, three fibrous bands (/) are drawn ; alternating with them is the soft bast, consisting 

 of sieve-tubes s, of granularly dotted (cambiform) cells, containing abund.int starch and protoplasm, and of other, 

 somewhat wider elements, bordering on the fibres, distinguished by clear watery contents and pitted walls. 



multiseriate zones of radially arranged sieve-tubes 2. In the old root, but not in the 

 stem of the White Fir, I often find, between two radial rows of sieve-tubes, radial 

 bands of parenchyma, which are uniseriate and resemble the medullary rays, but do 

 not lie in the same straight line with the medullary rays of the wood. 



* Hartig, Forstl. Culturpfl. p. 95, Taf. 9, 10. — Von Mohl, I.e. p. 891. — Graf zu Solms-Laubachi 

 Botan. Zeitg. 1871. 



^ Von Mohl. /. c. — Hartig, I.e. pp. 13, 35, Taf. 5. 



