1. Trichosanthes anguina is favorable for a study of the differentiation of the vascular bundle, because the phloem can be recognized very early and the long internodes make it easy to get straight views of the differentiating structures. 2. The spiral vessel of the protoxylem in its early stages has bands of peripheral cytoplasm which precede the spiral markings and have the same arrangement, and become the basis of the lignified spiral. The position of the cytoplasmic bands is determined by rows of vacuoles in the cytoplasm immediately preceding and during the formation of the cytoplasmic bands. The process of the early differentiation of the marking is a very rapid one. The position of the bands of the annular vessel is also determined by vacuoles, but the determining vacuoles are much larger than those of the spiral vessel. 3. The young cell of the metaxylem also has vacuolated peripheral cytoplasm which precedes the formation of its markings. The scalariform vessel, composed of long cells, is the fir...