The ontogeny of the xylem vessel from procambial cell was studied with the aid of electron microscope. The ultrastructure of procambial cells is made of highly dense cytoplasm with abundance of endoplasmic reticulum and Golgi apparatus with numerous vesicles. Moreover, dark-staining free ribosomes completely fill the cytoplasm. Microtubules can be detected along the cell wall. At a later stage of development, they appear to be localized. At this stage secondary wall thickening appears in the form of small electron transparent hemispherical humps in longitudinal sections of the procambial cells, and the microtubules are localized only in this region. In tangential sections it has been found that some Golgi vesicles lie in close proximity to microtubules, and sometimes the vesicles are completely surrounded by microtubules. From a series of electron micrographs of the primary xylem cell, at various stages of development, it has been concluded that the microtubules are instrumental for localized thickening of the tracheids.