通过对7种形态Zn,Cd与有效态Zn,Cd(DTPA提取）的相关分析及各形态Zn,Cd与植株Zn,Cd含量的通径分析，评价各形态Zn,Cd的生物有效性及对植株的相对贡献和途径。结果表明，交换态Zn,Cd与有效态Zn,Cd呈极显著正相关（r=0.954,0.953)，交换态Zn,Cd与植株Zn,Cd含量的通径系数为1．267、1．168，交换态Zn,Cd有效性高，具有最大的生物活性，对植株Zn,Cd含量的贡献最大。其它形态如碳酸盐结合态锌、氧化锰结合态Zn、Cd通过交换态作用于植株的通径链系数（间接通径系数）分别为0．856、0．592、0．723，上述形态对植株Zn,Cd含量也有影响，具有一定的生物相对有效性，但通过交换态间接作用。交换态不仅自身具有最大的生物有效性，而且还作为其它形态Zn,Cd生物有效性的中介，为紫色土其它Zn,Cd库流向植株的主要通道。

Correlation analysis between seven forms and available amounts (extracted by DTPA) of Zn and Cd, and their path analysis from soil to lettuce {Lactuca sativ L. )were conducted to evaluate bioavailability of zinc and cadmi-um and their contributions to the plant. The results showed that exchangeable Zn and Cd were significantly cor-related to available Zn and Cd ( r = 0.954 and 0.953) at significance level of 0.01, and path coefficients of exotic Zn and Cd to tissue Zn and Cd were 1.267 and 1.168 respectively, indicating the high bioavailability of ex-changeable Zn and Cd in purple soil. Exotic Zn and Cd made the most contribution to the plant, while the path chain coefficients of CAB-Zn, MnO-Zn, and Cd to the plant by indirect action of exchangeable forms were 0.856,0.592,0.723, respectively, showing that they may make some other contributions to tissue Zn and Cd through exchangeable forms. Therefore, exchangeable Zn and Cd are not only highest available, but also are bioavailable bridges and main paths from purple soil to the plant.