After military service as assistant surgeon in Maubeuge and Strasbourg and as surgeon and pharmacist in Algeria, Garreau became a professor of natural history at the University of Lille in 1844. His earliest researches dealt with the relative values of both surfaces of a leaf as sites of gaseous exchange, especially the exhalation of water vapor. He measured the amount of water excreted by placing small glass domes, containing a waterabsorbing substance, on opposite sides of a leaf and found no direct correlation between the amount of evaporation and the number, of stomata. He concluded that the epidermis determines transpiration, the cuticular layer being very important. On the veins, where transpiration was the most intensive, he found almost no cuticle.
Garreau also confirmed that leaves are able to absorb water, as Bonnet had proposed in 1754, and determined the osmotic properties of the epidermis and cuticle. Here he observed a more direct correlation between the exhalation of carbon dioxide and the number of stomata. Garreau discovered that there was no relation between the cuticle and the cells of the epidermis. The cuticle already exists before differentiation of the epidermal cells occurs.
Garreau also worked on the theory of respiration and nutrition of green plants, proposed by Ingen-Housz in 1779. In 1851 he confirmed the results of the work of H. B. de Saussure, who had shown that the great mass of the vegetable body is derived from the carbon dioxide of the atmosphere and the constituents of water. Garreau observed that reduction of carbon dioxide, which he called the nutritive function, was dependent on light and was independent of respiration. Although not strictly separating the effects of assimilation and respiration. Garreau protested against distinguishing a “diurnal” and a “nocturnal” respiration in green plants.
Garreau thought that the cuticle was a living tissue and that the younger the organ producing the cuticle, the stronger its osmotic activity. Further, Garreau believed that there was a direct relation between respiration and heat production. The idea of a vital force within the plant body was deprived of one of its chief supports when it was recognized that the natural heat of organisms is the result of chemical processes induced by respiration. Garreau explained the high intensity of this phenomenon in Arum inflorescences by showing that the surface area is large in relation to volume.
Garreau was a member of the Académie des Sciences and the Institut Impérial des Sciences.
Garreau married in 1846. He had four children.