CYTOPLASMIC ORGANIZATION AND RHYTHMIC STREAMING IN GROWING BLADES OF CAULERPA PROLIFERA

Light- and electron-microscopic studies of the growing blades and their meristematic tips in Caulerpa prolifera have been correlated with time-lapse photographic studies. The growing blade may be divided into three zones based on the level of maturation. A “meristemplasm” is present at the tip of the growing blade and at sites of wounding. The cytoplasm of these growing regions has an abundant endoplasmic reticulum, Golgi bodies, amyloplasts, and nuclei, but lacks chloroplasts and a central vacuole. An intermediate zone lies between the white meristemplasm and the green, mature basal zone. The basal zone contains a parietal cytoplasm with organelles typical of the Chlorophyta and a dominant central vacuole. Two distinct systems of cytoplasmic streams occur in the basal zone and both contain packets of microtubules orientated parallel to the axis of the streams. As the blade matures and growth ceases, the dominant central vacuole forms up to the tip. In developing blades the tips become white in the absence of light as a result of basipetal movement of the chloroplasts. In plants kept on a 12-hr L: 12-hr D cycle, blade growth and chloroplast migration at the blade tip are rhythmic, and the peak occurs about 2-4 hr after initiation of the dark phase. Experiments are reported using continuous light or darkness after three 24-hr periods of 12-hr L: 12-hr D phasing.