DNA, RNA, protein and heterochromatin were measured cytophotometrically in developing soybean (Glycine max) seeds. The average 2C DNA content for the soybean genome was 2.64 pg. The amounts of nuclear DNA in embryo axes showed no significant change during embryo development, whereas the DNA content in cotyledon nuclei increased significantly from 3.58 pg to 5.49 pg. The number of endopolyploid nuclei increased from 26% to 48% and the DNA content from 4.45 to 5.49 pg after cessation of cell division. The changes in RNA and protein content during embryo development were in general similar to those in DNA content. This can be interpreted that increased DNA levels in soybean cotyledons generated during embryogeny increase the protein synthesizing capacity. During the first 15 days of germination, the number of endopolyploid nuclei in cotyledons declined from 46% to 4%, and this decline is interpreted as DNA degradation providing a ready source of nucleosides and phosphates during early embryo growth. A later decline, however, between 15 and 20 days after germination, was age related similar to leaf senescence, because the percentage of endopolyploid nuclei remained unchanged while the number of non-viable cells increased. In senescing cotyledons, 73% and 80% of RNA and protein but only 20% of DNA were lost, as compared to dormant cotyledons. The heterochromatin (condensed chromatin) measurements indicated that nuclei of metabolically inactive dormant and senescent cotyledon nuclei contained an average of 33% more heterochromatin than nuclei from the green cotyledons of seedlings.

The base composition of peanut (var. NC-17) DNA determined from thermal denaturation profiles showed an average guanine plus cystosine content of 34% which was in close approximation to 36% guanine plus cytosine calculated from the buoyant density. Buoyant density also indi-

Cytophotometric measurements of DNA quantity per cell from 45 different Pinus rigida trees obtained from 135 slides showed that an internal standard was necessary in order to avoid serious errors. Chicken red blood cells were incorporated on the same slide as the pine cells, permitting the calculation of the actual mass units of DNA per pine nucleus. The use of these units makes the results of different investigations and investigators directly comparable. Staining variation between slides and staining batches was shown to be the reason for the requirement of an internal standard. DNA quantity in Feulgen units varied as much as 2.9 fold between different slides of red blood cells although the actual quantity of DNA is constant at 2.8 picograms per nucleus.

Nuclear DNA contents of a conventionally inbred flue-cured tobacco (Nicotiana tabacum L.) cultivar. 'Coker 139,' and two selected, doubled haploid (dihaploid) lines developed from it by anther culture and colchicine chromosome doubling were compared. The dihaploid lines had previously been shown to be lower yielding and agronomically inferior to the parental cultivar. Nuclei of leaf and root cells of the dihaploids possessed significantly higher amounts of DNA than the parental cultivar, although chromosome numbers (2n = 48) had not changed. Mean nuclear DNA content of the dihaploid lines was 10.62 pg, while nuclei of the parental cultivar contained 9.32 pg. The anther-culture process and (or) the nature of vegetative nuclei of pollen grains from which haploid plants originate have been suggested as possible contributing factors to higher DNA values. An average 12% increase in heterochromatin (condensed chromatin) in dihaploids, and significant differences in thermal denaturation of parental and dihaploid DNAs, suggest an amplification of DNA sequences during dihaploid formation.