Acta bot. bras. 21(3): 531-538. 2007 1 Departament of Botany, Aligarh Muslim University, Aligarh-202002 (U.P.), India 2 Corresponding Author: moin_a_khan11@yahoo.co.in; b.siddiqui@rediffmail.com Size variation in the vascular cambium and its derivatives in two Alstonia species Moin A. Khan 1,2 and M. Badruzzaman Siddiqui 2 Received: March 13, 2006. Accepted: November 21, 2006 RESUMO – (Variação em tamanho do cambio vascular e seus tecidos derivados em duas espécies de Alstonia). Duas espécies de árvores tropicais (Alstonia venenata Br. e Alstonia neriifolia Don. - Apocynaceae) foram estudadas para detectar variação em tamanho de diversos elementos do cambio e seus tecidos derivados. Embora as condições de clima e edaficas destas duas espécies fossem identicas, as dimensões das iniciais fusiformes e os elementos derivados destas foram maiores em A. venenata do que em A. neriifolia. As iniciais radiais são retangulares em A. venenata porem são isodiamêtricas em A. neriifolia. Foi observado aumento substancial no comprimento das células do floema e xilema quando comparadas com as células mãe. Alongamento máximo foi observado nas fibras do xilema durante a diferenciação das respectivas iniciais fusiformes. Palavras-chave: comprimento das iniciais funsiformes, iniciais radiais, elementos do tubo crivado, elementos de vaso, fibras do xilema, Alstonia venenata, Alstonia neriifolia ABSTRACT – (Size variation in the vascular cambium and its derivatives in two Alstonia species). Two tropical tree species viz. Alstonia venenata Br. and Alstonia neriifolia Don. (Apocynaceae) were investigated to detect size variation in different elements of the cambium and its derivative tissues. Although these two species were grown under identical climatic and edaphic conditions, fusiform initial dimensions and the elements derived from them were larger in A. venenata than in A. neriifolia. Ray initials are rectangular in A. venenata but isodiametric in A. neriifolia. An appreciable increase in length was observed in the phloem and xylem ray cells when compared to the mother cells. Maximum elongation was observed in xylem fibers during differentiation from the respective fusiform initials. Key words: cambial fusiform initial length, cambial ray initials, sieve-tube elements, vessel elements, xylem fibers, Alstonia venenata, Alstonia neriifolia Introduction Information on size variation of cambial initials and their derivatives is rather meager. The pioneering contribution in this field is by Chattaway (1936), Butterfield (1973) and Anand et al. (1978) who studied cambial initials and their derivatives in Ginkgo, several conifers and dicotyledons. Recently Iqbal (1990), Rao et al. (1996), Paliwal & Yadav (1999) and Paliwal et al. (2002) also observed the structural and size variation of different xylem elements in Leucaena leucocephala and Haldina cordifolia, respectively. In three dimensional view, the cambium is a continuous cylindrical sheath about the xylem. There are two conceptually different views regarding the nature of cambium. One school of thought postulates a multiseriate zone distinguished in transections by radially narrow cells with thin walls in which all the cells are equally endowed with multiplication capacity. This view, proposed by Raatz (1892), has been strongly supported by Catesson (1964). The other school pleads for the uniseriate nature of cambium. There are two interpretations of this uniseriate concept based on terminological differences. According to one, there are single initial cells which in each radial file of cambial cells lie somewhere between the phloem and xylem mother cells and are responsible for the production of cambial derivatives on the outer and inner sides. This view is mainly advocated by Bannan (1955; 1968) and Newman (1956), and has been supported by ultra- structure studies of Mahmood (1968) and Murmanis (1970) pertaining to tangential wall characteristics. According to another group of workers (Wilson et al. 1966; Zimmermann & Brown 1971), the term cambium is applicable only to the initial cells, not the immediate derivatives. Following the former terminology, Butterfield (1975) defines cambium as a “multiseriate zone of periclinally dividing cells lying between the