By 1903 Bateson had begun work on sweet peas and these records are contained in a series of 40 notebooks, 17 of which are presented here. The start of this series of experiments coincides with the rediscovery of Mendel’s papers on sweet pea genetics, which clearly triggered Bateson’s interest in sweet peas. Sweet peas were particularly suitable for genetic studies due to their ability to self-pollinate and their easily observed Mendelian traits, including colour, height and petal form.

As with the poultry breeding, Bateson began with commercial varieties of sweet peas, which showed variation in plant form and flower colour.  Many of these varieties were obtained from seed companies, in particular Suttons of Reading, and are named in the early records.  One of these, Blanche Burpee, is a true breeding white variety, which when crossed with another white variety, Emily Henderson, produced progeny with purple flowers and led to the discovery of complementation.

The sweet pea notebooks are labelled on the cover with the year, and inside with ‘sweet pea crosses’ and sometimes where the plants were grown.  For example, 04 F  contains records of plants grown ‘at the farm’. This could perhaps mean Trinity farm, although later notebooks in this series refer to Merton Hall Farm, and this may be a more likely location. There are clearly at least two handwriting styles in these records, and since Bateson and Punnett had begun their collaboration, it is probable that at least one of these is Punnett’s.

The general format of the record is that the parents are recorded as numbers with a superscript, e.g.  90 2 x 68 1, or sometimes a named variety (e.g. Blanche Burpee). Since sweet peas can be selfed, some crosses have only a single parent. The date of sowing and the date of planting out are also recorded, together with other observations on the plants such as colour, height and petal form.  In the earlier books these observations are cryptic and difficult to decipher, with lots of abbreviations used (e.g. rd = red; pk = pink; wh = white; prpl = purple; lt = light; dk = dark; EH = Emily Henderson; ax = axil; pic = picotee).

Some plants were grown in the open in farm plots, while others were grown in pots.

Seeds kept are given a number, generally referring to the page of the notebook, with a superscript for the individual seed. At a later date these identifying numbers were modified to a different format e.g. 100/12, and these modifications are recorded in blue, red or green pencil next to the original.

Two of the key books provide summaries of sweet pea crosses - MS Add.10161/1/5/16  for 1904-1909 and 1914 and MS Add.10161/1/5/17  for 1908-1939. The latter is labelled ‘ Lathyrus ledger’, signed inside by R.C. Punnett, Whittinghame Lodge. It is important because it contains summary records of the sweet pea crosses carried out from 1908 through, in some cases, to 1931, with additional notes for 1936-1939.  These summaries are cross referenced to the main series of sweet pea notebooks.  Inside the cover of this ledger is a list of letter codes  referring to phenotypic characters, e.g. A1 - purple/red (flowers), A2 - long/round (pollen), A3 - erect/hooded (flower shape). Many of the crosses in the notebooks are then identified only by these codes e.g. A1 x D2.

Some books contain items of particular note, and the following are just a few examples:

From Book 06 II onward, many of the notebooks contain maps of the plots where the plants were set out during the summer.

Book 08 contains details of a cross they were clearly looking for: a 3:1:1:3 Mendelian segregation. From this point on the scoring and classification of progeny becomes much more detailed and systematic, and tables of classified progeny begin to appear.

Book 10F contains the first instance where the term ‘F1’ is used to denote the first filial generation. Experiments were being carried out on a bigger scale and more progeny were being scored (150 to 200 per cross in some cases).

In Book 10 + 11 there is a note on the fly leaf , ‘All plants saved for seed at Merton died from ?eelworm; W.B.’  This could refer to Merton in Surrey, which was then the location of the John Innes Institute, or perhaps to Merton Hall farm, Cambridge, which was on the site currently occupied by the Vet School and Computer Labs, off Madingley Road.