Fertilisation of Orchids (1862)
The Movements and Habits of Climbing Plants (published as an essay in 1865, and as a book in 1875)
The Variation of Animals and Plants under Domestication (1868)
Insectivorous Plants (1875)
The Effects of Cross and Self Fertilisation in the Vegetable Kingdom (1876)
The Different Forms of Flowers on Plants of the Same Species (1877)
The Power of Movement in Plants (1880)
The Formation of Vegetable Mould through the Action of Worms (1881)
I found this site which discusses Darwin's interest in plants: https://ncse.com/library-resource/charles-darwin-botanist The author writes:
|An 1891 printing|
"Darwin’s love of plants appears to have been deeply rooted in his childhood. His parents were both interested in gardening and maintained a varied collection of plants in their conservatory and gardens in Shrewsbury, where Darwin grew up. Indeed, one of the few images of Darwin as a child (age 6) show him kneeling with a potted plant on his thigh. In his autobiographical chapter, Darwin (1887) mentions that '…apparently I was interested at this early age in the variability of plants!' A schoolfellow remembers Darwin’s bringing a flower to school and saying that his mother had taught him how to identify the plant by studying the flower."
Another site, http://www.bgci.org/education/article/0659/, is a discussion of Darwin's botanical work. Both of these articles are worth reading as a whole. This author notes:
"Darwin wrote six botanical books, over 75 articles, and well-articulated and rigorously executed studies. He titled his last plant book The power of movement in plants (1880). Fascination with the ways that plants move entered the early botanical literature in conjunction with a difficult question: Are plants alive? Although they don’t have evident nervous systems, they can respond to irritation in some cases. This was taken as a sign of life. At the bottom is the even more difficult question, ‘What is life?’ For Darwin, the traditional ‘What is life?’ question was transformed into an effort to demonstrate the unity of all life and hence, by implication, the common descent of all branches of the evolutionary tree. This is the unspoken thesis of Power of Movement. Darwin was once again universalising. By showing that plants have a power of movement and given that mobility and the capacity for movement are animal-like characteristics, he is supporting the unity of common descent, which in turn is an underlying assumption or implication (depending on how the argument is phrased) of evolution...
"As we have seen, Darwin not only contributed to botany, he actually changed the discipline by his very contributions. Since all of his botanical researches were conducted as applications of the theory of evolution, replete with well-worked examples often treated as evidence for natural selection, he was using botany to defend his theory. But simultaneously he was also providing botanists with a model for how to think about their own observations in evolutionary terms....Darwin was so invested in his plant research that he himself built the bridge directly from the Origin to fundamental problems in botanical science. Thus did Darwin assist at the birth of evolutionary botany. This dual role as both founder of evolution by natural selection and exponent of how the theory could be applied in botany is quite remarkable. Later generations of Darwinian botanists would complete the transformation of botany into evolutionary science. But Darwin went a long way in providing a model for that transformation."
(Among his other, varied investigations, Darwin was also interested in marine invertebrates, and wrote articles and books on the subject, particularly barnacles, during the years prior to On the Origin of Species. Here is an article that discusses his research: http://darwin-online.org.uk/EditorialIntroductions/Richmond_cirripedia.html )
A friend recommends Janet Browne's biography of Darwin.