The habit of long and continuous reflection on fundamental problems, which was so marked a feature of Brown's character, was perhaps responsible for the curious manner in which some of his most valuable and suggestive contributions to science, and especially to morphology, were given to the world, a habit to which I have already adverted.
We know he had been for many years interested in the ovule, and he made a number of important discoveries respecting it. Closely bound up with this topic were his studies on the Cycads and Conifers. He observed the plurality of embryos in the seeds of these plants, and, indeed, makes a reference to the phenomenon of polyembryony in the Prodromus, in which, as in most of his systematic works, morphological observations of the highest value are scattered, though embodied in very compressed phrases, amongst the descriptions of species. But every now and then when writing on one subject he seems to be carried away with the rush of his ideas on general questions. Thus in a memoir on the genus Kingia he entitles the paper, possibly to save his face after he had written it, "Character and Description of Kingia; a new genus of plants found on the south-west coast of New Holland. With observations on the Structure of its unimpregnated Ovulum, and on the female flower of Cycadeae and Coniferae."
This paper is, perhaps, one of the most important of his works, for it was there that, having briefly dismissed the genus Kingia, he "let himself go" on the ovule, and then in a masterly dissertation, puts forward his view on the gymnospermic nature of the Cycads and Conifers.
He summarises what was known at that time as to the structure of the ovule, acutely criticising the views of the various authors he cites. He emphasises the need of studying the development in order successfully to interpret the mature structure. He insists on the origin of the seed coats from the integuments, on the orientation of the embryo within the amnios (embryo sac), and on the distinction between the true albumen which is contained in this "amnios" and the albumen "formed by a deposition of granular matter in the cells of the nucleus" (nucellus), i.e. the perisperm, and he goes on to suggest that in some of these cases the "Membrane of the amnios seems to be persistent, forming even in the ripe seed a proper coat for the embryo.... This is the probable explanation of the structure of true Nympheaceae" ... here he seems to have overlooked the rudimentary endosperm which is really present. Finally he sums up an admirable account of the whole matter as follows:—"The albumen, properly so-called, may be formed either by a disposition or secretion of granular matter in the utriculi of the amnios, or in those of the nucleus itself, or lastly that two substances having these distinct origins and very different textures may coexist in the ripe seed as is probably the case in Scitamineae."
He then goes on at once to argue that the apex of the nucleus is the point of the ovulum where impregnation takes place, and adds that "all doubt would be removed if cases could be produced where the ovarium was either altogether wanting or so imperfectly formed that the ovulum itself became directly exposed to the action of the pollen or its fovilla." This leads him at once to enunciate his view of the gymnospermy of Cycads, Conifers and Gnetaceae. He reviews very fully the opinions that had been expressed by others as to the real structure of the female organ, especially of Pinus, and he mentions the fact that he himself in the botany of the Flinders' voyage had previously held the view that a minute perianth was present in the Pine, a view which, as he says, "On reconsidering the subject in connection with what I had ascertained respecting the vegetable ovulum" he had now abandoned.
The morphology of the male sporophyll of Cycas, however, presents a great difficulty, and Brown, less fortunate here, discusses a number of what seemed to him possible explanations. The recognition of Sporangia was remote, and the effort to homologise the numerous pollen sacs either to grains of pollen which, bursting, liberated fovilla, or to male flowers, or to explain them in other ways, was not very successful. The fact is this was a piece of morphology for which the age was not ready. We must recollect that the comparative morphology of the ovule (in the wide sense) was not attempted. Brown's main contribution to the understanding of this structure consisted in the empirical accuracy with which he elucidated the actual structure—he made no attempt to frame a comparative morphology, for the simple reason that in the condition of knowledge at the time no such comparative morphology was possible or even dreamed of.
Two other remarkable discoveries now demand our attention, and both are instructive as shewing the keenness with which his highly trained powers of observation followed up the clues which his brilliant intellect had enabled him to descry. It was while engaged on a study of the Orchids and Asclepiads that he was led to recognise the existence of the cell nucleus. He worked almost exclusively with what we should call a dissecting microscope. One of his instruments is preserved in the Natural History Museum, and it is well to examine it and reflect on how much may be discerned even with a very primitive instrument if only a good brain lies behind the retina. The "microscope" contains a number of simple lenses of various powers, the highest about 1/32" F.L. It is easy with such an instrument to see the nucleus in the epidermal cells when one knows it is there, but to have discovered it, and at a time when the technique of staining, &c., was simply non-existent, was a triumph of genius. Brown, of course, could not fully appreciate the great importance of his discovery, but he quite realised that he was dealing with no isolated or trivial fact, and, with characteristic industry and enterprise, he searched many other plants to find out whether his newly recognised nucleus was general or not; he found it to be so, and we all know how the discovery began at once to bear fruit.
A second observation to which I would refer was also of wide interest, and it was not made merely by chance. Brown was anxious to penetrate if possible into the secrets of fertilisation. He seems to have been pretty sure that something more than the mere "aura" of older writers was concerned in the matter, and while looking into the evidence for the existence or transmission of material substance, he observed that in the fovilla of the pollen there were vast crowds of minute particles which were in a continual state of dancing motion. He hoped that it might be possible to identify these bodies along their track into the ovule, and so to settle the more urgent questions as to the mode of fertilisation. He states that he made his observations with a simple microscope, the focal length of the lens of which was 1/32". Later on he used a much more powerful pocket microscope made by Dollond with power up to 1/70" F.L. He got Dollond to check the results with a compound achromatic microscope, and estimated the size of the particles to be 1/20,000 to 1/30,000". Brown was fully aware that he was not the first observer who had seen these moving particles. They had been already noticed by Needham and by Gleichen, but these writers had paid no special attention to them. Brown's great merit in this matter lies in the admirable way in which he conducted the investigation. At first he thought he had lighted upon something which was essentially a peculiarity of the male elements; then, extending his observations, he had to expand his first idea and admit the "active molecules" to represent a state or condition of living matter generally. As he still further widened the sphere of his investigations, he proved that the same movements occurred in dead tissues, and further that inorganic bodies also exhibited the phenomenon. Later on he found that the movements depended on the minuteness of the particles. He excluded the effect of evaporation, currents and other disturbing influences, and, indeed, the whole investigation shews him to us in the character of an accomplished experimenter as well as a brilliant observer. The complete explanation of these "active molecules," which are in the state generally described as "Brownian movement," still constitutes an unsolved problem, and one finds that it even now continues to occupy the attention of the physicist.
Any attempt adequately to review the whole of Brown's life work is impossible within the limits necessarily imposed by the conditions of a lecture, and I make no pretence to completeness, but will endeavour rather to indicate what appear to be the more important of his many other contributions to science.
His catalogues of the plants collected by those associated with various expeditions, his Kew lists (which were published under Aiton's name) are well known to students of systematic Botany, but his fine monograph on Rafflesia, containing, as it does, many observations of general interest will well repay perusal even after these many years. His studies on Cephalotus, on Caulophyllum (with its remarkable seed formation), as well as his considerable memoir on the Proteaceae, shew him as a naturalist imbued with keen insight and possessed of extraordinarily sound judgment.