A Fourier transform infrared (FT-IR) spectrometer equipped with a microscopic accessory was used to monitor the cellulose crystalline structure at each developmental stage of coniferous tracheid cell wall formation. The spectra showed that the cellulose in the primary cell wall was rich in the metastable I-alpha crystalline form and interestingly had a higher crystallinity than the secondary wall cellulose composed mainly of the stable I-beta crystalline phase. These results indicate the presence of an in vivo physical force at the cell surface which may stress the primary wall cellulose during the crystallization. Moreover, the primary wall cellulose was oriented parallel to the enlarging direction of growing cells. Thus, we consider that the cellular growing stress generated between the plasma membrane and the primary wall may elongate just-biosynthesized beta-glucan chains and cause them to crystallize with a higher crystallinity of the metastable I-alpha phase, as is the case for crystallization occurring during the drawing of polymer gels.