Structure and Behaviour of Organelles in Living Pollen Tubes of Lilium longiflorum

The morphology and the movement of organelles, in particular of the endoplasmic reticulum (ER), have been studied using video-enhanced microscopy in living pollen tubes of Lilium longiflorum Thunb. The patterns of the cytoplasmic streaming in the pollen grains and tubes generally correspond with the descriptions by Iwanami (1956). We found, however, that the organelles move as single and independent elements reflecting highly individual characteristics. The apical zone of the pollen tube, the so-called ‘clear’ zone, is populated by numerous intensively vibrating organelles of small size, most probably Golgi vesicles. The migration of vesicles to the apical plasma membrane of the growing pollen tube is not a smooth flow of particles, but a turbulent process. Spherical bodies with a high contrast, probably polysaccharide (p-) particles and/or lipid drops, and some mitochondria are also visible in the extreme tip. Their penetration into the apex seems to be inhibited by the dense mass of agitated vesicles. The small vacuoles located in the central zone are very motile; they change their shape continuously. The tonoplast, therefore, must be very plastic and, to a certain extent, mechanically resistant. The vegetative nucleus and the generative cell maintained a stable shape despite constant collisions with the organelles of the vigorously streaming cytoplasm. The thin cytoplasmic layer in the vacuolate zone contains tubular ER (diameter about 0.3 μm) arranged in complex closed networks. The tubules are permanently moving. Gentle undulations alternated with sudden dramatic distortions. Rapid fusions or divisions of tubules are common. This dynamic behaviour is taken as proof of the plasticity of the ER tubules and the high fluidity of the membranes of the ER. Organelles preferentially move along the ER tubules in a saltatory manner. It is assumed that the ER tubules are associated with cytoskeletal elements.