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Summary After impregnating root meristems with silver nitrate two types of small (< 1 ?m diameter) body can be seen in the nuclei. These have been termed “dense body” (DB) and “nucleolus-associated body” (NAB). The number of these bodies within a nucleus varies from species to species, but in general DBs are relatively numerous and lie in the nucleoplasm, while the NAB is usually solitary and lies on the surface of the nucleolus. Using nuclear volume as an indicator of the age of the nucleus since mitosis, the numbers of DBs and NABs were related to the nuclear growth cycle. In the meristem ofPisum sativum andZea mays DBs are characteristically present in early interphase; in some regions they persist in the nucleoplasm until the next mitosis, in other regions they disappear during interphase. DBs are probably pieces of the pellicle of ribonucleoprotein that coats mitotic chromosomes which have not coalesced (as does the remainder of the pellicle) to form the nucleolus at the start of interphase. NABs grow out from the nucleolar surface during the later stages of interphase. At the end of interphase there is on average 1 NAB per nucleolus.

Summary Nuclear bodies are found in interphase nuclei of root apices of a number of plant species. They often show differences in structure and position relative to the nucleolus and this has led to an attempt to define two classes of body. However, in some species their separation into two classes on structural grounds alone breaks down, indicating that although they may occupy different positions within the nucleus they may in these particular cases be only different forms of the same body. The two extremes of the range of bodies examined represent what have been called “nucleolus-associated body” (karyosome) and “dense body”. The nucleolus-associated body is typically attached to, or adjacent to, the nucleolus. It is composed of fibrils 4–8 nm wide and often has an open structure showing compound threads or fibrils separated from each other by electron-lucent spaces. The dense body is more compact in structure and typically lies free in the nucleoplasm. Both types of body have an affinity for silver ions which, together with their staining reaction following treatment with EDTA, indicates that they consist of ribonucleoprotein. The characteristics of nuclear bodies found in different plant species have some relationship with the structure and DNA content of the interphase nucleus. Nucleolus-associated bodies are characteristic of species with an areticulate nuclear structure (2 C DNA content <6 pg), while dense bodies are common in species with a reticulate nuclear structure (2 C DNA content >6 pg). The possible functions of the two forms of nuclear body are discussed.

Summary The karyosome is a spherical body up to 1 µm in diameter that lies on the nucleolus of certain plant species, particularly those with a relatively low nuclear DNA content and an areticulate nuclear structure. It can be seen in the light microscope after impregnation with silver; in the electron microscope its structure consists of fibrillo-granular material. Nucleoli of cells in root apices may bear 0, 1, or 2 karyosomes. The frequency with which these numbers of karyosomes are observed depends on the location of the cells within the apex. In roots ofPisum sativum andZea mays the nucleoli of both slowly-dividing and young differentiating cells bear karyosomes more frequently than the nucleoli of rapidly-dividing cells. The karyosome seems to adopt a preferred location on the nucleolus, lying most frequently on the nucleolar surfaces directed towards the apex or base of the root. The origin and functional significance of the karyosome are discussed. Morphological evidence suggests that it may be material that formerly was part of a fibrillar centre.

Summary Selected nuclear and cytoplasmic changes associated with early differentiation of four cell-types—dermatogen, inner and outer cortex, and endodermis—have been analysed using montages of electron micrographs of median longitudinal sections of young roots ofAzolla pinnata. The area fraction of nucleoplasm occupied by chromocentres (CAF) is smaller in the apical cell than in the nuclei of its most recently formed daughter cells. The CAF also differs between the four cell-types: dermatogen nuclei have a lesser mean CAF and smaller chromocentres than nuclei of the endodermis; cortical cell nuclei have intermediate values. These differences may reflect changes in nuclear activity during cell differentiation. The area fraction occupied by the vacuome (VAF) differs between the apical cell and its daughters: the apical cell seems to retain most of the vacuome at division, while the daughter cells receive less vacuolate cytoplasm. Of the four cell-types analysed, the cortical cells develop a large VAF the quickest; the dermatogen is slower to become vacuolate. Cells in the dermatogen and outer cortex derive from common mother cells, as do cells in the endodermis and inner cortex, and even the most recently-formed cells in the files of inner and outer cortex are more vacuolated than their sister cells in the other two celltypes. The onset of vacuolation may be triggered by an inductive influence emanating from older vacuolated cells in the same file. The rate of vacuolation in each of the cell-types examined may also be negatively correlated to the intensity of synthesis of protein used to construct cytoplasmic materials.

Summary The growth of the nuclei and cells that constitute the hairs on the anther ofBryonia dioica has been investigated. The cell at the base of the hair reaches a volume of nearly 106 µm3 just before the flower opens, while the cells distal to it do not grow and have volumes of less than 104 µm3. The nucleus of the basal cell undergoes endopolyploidisation, doubling its DNA every 1.3 days in the early stage of this process, to reach a maximum DNA content of 256 C which is equal to 512 pg of DNA. Nuclei in the distal hair cells do not have more than a 4 C DNA content. The endopolyploid nucleus of the basal cell contains chromosome structures that are polytene. Homologous polytene chromosomes often appear to be paired.

Summary The ultrastructure of the hair cells on the anthers ofBryonia dioica has been examined at two stages during their development. In the mature hair the cell at its base contains extensive rafts of endoplasmic reticulum while in the distal hair cells the ER forms a tubular network. In the basal cell the mitochondria degenerate and their contents pass into the cell wall; no such degeneration of mitochondria was observed in the distal hair cells. The plastids contain starch and an unusual membrane system of tubules and crescents. During hair development the wall becomes thickened with cuticular deposists. Many of the ultra-structural features of the cell contents could be accounted for by excessive growth of the membrane systems. There was no evidence that the hair cells manufacture or export any secretion.

Summary Optical density profiles of Feulgen-stained nuclei ofBryonia dioica at different stages of the mitotic cycle were determined. Nuclei in the G2 phase have a greater fraction of dense chromatin than nuclei in G1 phase. However, nuclei at the end of the S phase have dispersed chromatin of minimal density. Thus, chromatin density oscillates during the mitotic cycle of this species, consequently the progressive increase in density previously recorded throughout the intermitotic period of two other species (onion and mouse) cannot be a general rule.

Summary A small proportion of nuclei in root meristems ofAllium flavum, Bryonia dioica andLupinus angustifolius exhibit a dispersion of their chromocentres; the structure of such nuclei corresponds to the “Zerstäubungsstadium” (Z-phase) described byHeitz.

Summary A 3 hour exposure of pea roots to 0.025 per cent colchicine causes the cessation of root growth and ac-tumour to form. In roots allowed to regenerate it is found that in the region of thec-tumour, metaxylem tissue fails to differentiate completely, but protoxylem is unaffected. Development of secondary xylem tissue is also inhibited in the tumour region.