Fine structure study on the development of trabeculae in the siphonous green algaCaulerpa simpliciuscula C. Ag.

Summary Bundles of fibrils and tubular structures were found to be associated with growing trabeculae ofCaulerpa simpliciuscula. In the rhizome tips, the bundles had an average diameter of 0.1 to 0.3 m, and a length greater than 10 m. The fibrils in the bundles were oriented in a strictly parallel fashion, with an individual thickness of 3–8 nm. The development of trabeculae started with the apposition of material of low electron density onto the bundles, which in this way became the inner skeleton of the trabeculae.Although fibre bundles with the same internal structure also occurred in the frond tip, these rarely contributed to trabecula formation. In the frond tips a different type of bundle with paracrystalline structure was found associated with the trabecular surface, forming a temporary connection between adjacent trabeculae. Permanent connection was achieved by deposition of further layers of trabecular material. These bundles in the frond tip consisted of two layers of tubular elements with a wall thickness of 80 Å and an inner diameter of 20–25 nm.Both fibre bundles and tubular bundles appear to contribute to trabecula formation. The similarity of these structures to the vacuolar inclusions observed in other siphonous algae is discussed.