A plant species may have a mixture of autopolyploids and allopolyploids, and such species are called polyploid complexes. Polyploids are rare in animals but common in plants. Many plant species have been created via polyploidy.

Plant chromosome studies were not conducted in China until the l980s, a hundred years later than in some developed countries. After the reform and opening up in 1978, such studies were launched in the schools of biology in Nankai University and Peking University, the Institute of Biology of the CAS and the Chinese Academy of Agricultural Sciences. Now China's research in this field is close to or at advanced world level.

Chen's plant chromosome atlas was inspired by the research of Xu Daojue, a Chinese-American researcher. In the 1960s and 1970s, Xu collected samples of mammal organs and lymph cells from various places of the world, and analyzed their chromosomes. The result was recorded into an Atlas of Mammalian Chromosomes, which was first published in 1967. That was the first such atlas in the world.

Chen, then in his 40s, was encouraged by Xu's publication. In 1978, Chen started to collect plant samples and analyzed their chromosomes. Now, Chen, in his 70s, has published his atlas.

Chromosomes are thread-like structures in the nucleus of a cell. When the cell is not dividing, chromosomes are not visible. When the cell divides, chromosomes become visible under a microscope because they become more tightly packed. Researchers can observe chromosomes during cell division.

Each chromosome is made up of DNA, which has a double helix structure. DNA is usually double-stranded. Chemically, DNA consists of nucleotides. Two nucleotides on opposite DNA strands that are connected via hydrogen bonds are called a base pair. The chromosome atlas describes a gene's position on a chromosome based on the sequence of base pairs.

Chen and his colleagues took photographs of chromosomes through a microscope. They clipped chromosomes out of a photo and arranged them in sequential pairs, to produce a picture of the whole thing. From 2007, they have used microscopes with digital cameras, so that they can process the chromosome photos on a computer. Chen and his colleagues also invented methods to produce chromosome samples.

In addition to working in the laboratory, the research group often trekked over mountains and rivers to gather plants. Most wild plants grow in remote areas, and it is extremely difficult to pick them. "The ancestor of oranges grows in a remote mountain in Yunnan Province, which is one-week drive from Kunming, the provincial capital," Chen said. "Wild tea plants grow in virgin forest on the border with Myanmar. There was no road, so we blazed our own trail."

While compiling the plant chromosome atlas, Chen received help from hundreds of individuals and over 50 organizations. For instance, the three wild rice plants recorded in the book grow in remote mountain areas in the island province of Hainan. It was very difficult to locate these rare plants. Once, when Chen learned the location of a wild rice plant and rushed to the place by train, he found the field already built on. With help from local people, Chen eventually found the three wild rice plants recorded in the atlas.

As chromosomes can only be observed when cells divide, researchers have to collect and observe plants whose cells are actively dividing. This can be difficult as most of the time plants are dormant. Chen took horsebean as an example. In a 19.5-hour cycle, the cells are dividing in only two hours, while in the remaining 17 hours, chromosomes cannot be observed.

Plant gene groups usually have a large number of chromosomes. The gene groups of some plants have hundreds of chromosomes. Recording a large number of chromosomes is a challenging job. "Now, China has 50,000 plant species and the chromosomes of less than 20 percent are recorded in various literatures," Hong said.

Gene research is expensive. Chen and his team have received funding from several sources. He has also trained young people to work on his project. In the future, Chen hopes to work with computer scientists to develop mathematical models for the plant chromosome information collected.