1. In fully senescent mesophyll cells from Phaseolus almost all the cytoplasmic contents had been lost apart from the resistant plasmalemma and some small empty vesicles. In some parts of the cell groups of plastids, together with occasional distorted mitochondria, spherosomes and a few ribosomes also remained. Such plastids were much smaller than those of normal cells and had a spherical shape. The surface membrane had remained intact and enclosed a group of spherical globules with dense structureless contents. All the thylakoids and the stromal ribosomes and starch, shown to be present in the stroma of normal cells, had been broken down. The globules are formed by the accumulation of lipids from thylakoid breakdown, and contain the lipid-soluble carotenoids freed at the same time. 2. The earliest stage of senescence examined showed more or less intact cells apart from the accumulation of some globules within each chloroplast. Distortion of the thylakoids in the vicinity of these globules is considered to be evidence for their localized breakdown in the chloroplast. 3. Because the well-ordered sequence in early senescence it is thought that wholesale release of hydrolytic enzymes would not account for this. At a later stage however, wholesale release of materials from various organelles would constitute a mopping-up process. Of particular significance here would be the vacuolar contents. 4. It would seem from the findings of this investigation that the primary cause of the senescence sequence would be consistent with the appearance, either by synthesis or activation, of a specific enzyme complex which is capable of decomposing thylakoids in the chloroplast. It is thought that this enzyme would increase in quantity and move from the plastids to affect membranes around other organelles at a later stage.

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Phytoferritin-like particles were found within chloroplasts of senescing Phaseolus leaves. Large quantities of the particles were laid down at an early stage of senescence, and no further deposition occurred. All but a few groups of particles had been removed by the time the leaves turned yellow, this removal taking place at a late stage of senescence which coincided with the release of the vacuolar contents.

New intranuclear crystals were described in cells of Chara vulgaris. These were needle-like structures extending for about 8 μ in length and 350mμ×110mμ in transverse dimensions. Each crystal was composed of hexagonally-packed, tubular structures about 200-240 Å in diameter, apparently running parallel to each other along the longitudinal axis. Such crystals appeared to pass through one of the several nucleoli in each nucleus. It is suggested that the crystals are proteins having a storage function, which are synthesised in the nucleolus.

SUMMARYThe ascospores of Podospora anserina (Ces.) Rehm are delimited by a double membrane system. The primary spore wall develops within this, the outer part of the double membrane being pushed out to form the spore membrane and the inner part forming the plasmalemma of the spore. Starting in the middle of the matrix of the expanding primary wall, a secondary wall is laid down and gradually extends to the outer periphery of the spore wall. Later, a thick tertiary wall is formed at the inner side of the secondary wall by blocks of electron-dense material between which channels of the primary wall matrix remain. This is the pigmented layer of the spore wall. On the innermost side of the spore wall, a part of the original primary wall remains.The primary appendage at the base of the spore arises as part of the spore initial, but, after it has been cut off by a septum, its contents degenerate and it is bounded only by the primary and secondary wall layers. The secondary appendages, formed at the apex of the spore and at the bottom of the primary appendage, are considered to be actively growing processes bounded by the spore membrane.