This is how an alga (that’s how it’s spelled in the book) reproduces. It looks like an underwater nasturtium.
Here, it’s being used to demonstrate what exactly RNA is doing in cells when living things reproduce. Algae are nice and simple, so little changes can be easy to see. This particular water plant helped biologists understand that the cell nucleus is where all that reproductive information gets stored.
The book in question is The Biological Role of Ribonucleic Acids by Belgian biologist Jean Brachet. Not sure if Brachet did his own illustrations – most of them are photograps of cells way up close, so that they look like Star Trek planetscapes. But I like this little garden here.