(b. Châtillon-Colligny, Loiret, France, 8 June 1866; d. Paris, France, 24 July 1944)
For fifty years after his graduation from the École Normale Supérieure in 1894, Molliard taught and worked at the Faculty of Sciences of the Sorbonne; he was its dean for six years and had the first chair of plant physiology in France created for him. Deeply imbued with Lamarck’s ideas, he devoted all his writings to the influence of the environment on plants. He stated:
The most general idea which emerges from my studies is that plants, even the most differentiated, are extremely plastic, much more so than has been admitted until now, that their structure is closely dependent on their chemistry, the latter being influenced by external conditions; it is therefore an experimental confirmation that my researches contribute to Lamarck’s theory, insofar as its essential features are concerned [Oeuvres scientifiques, p. 6].
Molliard began by investigating the morphological transformations that certain parasites produce in plants and that lead to the formation of galls; his last works were concerned with the conditions of tuberization in the potato (Solanum tuberosum). In order to carry out precise studies, he controlled all the nutrients. He also eliminated all possible parasites by cultivating his plants, particularly radishes, in an aseptic environment from germination to fructification.
Molliard systematically investigated the mineral nourishment of the mold Sterigmatocystis nigra. Normally it lives on sucrose, which it transforms into carbon dioxide and water by respiration; only a very small amount of organic acids appears in the medium. If too little nitrogen is furnished, a large quantity of citric acid is produced in the medium. If too little phosphorus is supplied, the medium abounds in both citric acid and oxalic acid. If potassium is lacking, only oxalic acid is abundant. If all of the mineral elements are reduced, gluconic acid appears. Hence, well before Hans Krebs discovered the acid cycle named for him, Molliard drew attention to the importance of organic acids in intermediate metabolism.
Molliard’s studies on the radish are famous. Cultivated asepticaliy, provided with light but in an atmosphere without any carbon dioxide—and consequently without the assimilation of chlorophyll— the plant absorbs glucides through its roots. If the supply of glucides is abundant, the form of the radish is altered; the glucides no longer accumulate in the tissues as sucrose or monosaccharides but as starch. The reserves, instead of remaining in the root, move into the stem, which swells and acquires the characteristics and appearance of a subterranean stem. The organic nutrition totally transforms the physiology of the plant and thus modifies its microscopic appearance and morphology.
Through multiple experiments of the same kind Molliard showed that it is possible, by varying only the nutrition, to transform ordinary leaves into cotyledons. In addition, certain leaves can be changed into thorns and certain thorns into leaves. He also showed that parasitic plants like the dodder (Cuscuta), which ordinarily feeds on clover and alfalfa to which it attaches itself by its suckers, are able, if they receive suitable nutrition, to live independently without suckers but with abundant chlorophyll.
Molliard was the leading authority on plant physiology in France. He spent the whole of his professional life in the same laboratory, on the second floor of the old Sorbonne building. From 1894 to 1940 all plant physiologists in France were more or less his direct pupils. His writings consist primarily of short notes to the Academy of Sciences, of which he was a very influential member.
Molliard’s most important book is Nutrition de la plante, 4 vols. (Paris, 1923). Among his many papers are the series with the general title “Recherches physiologiques sur les galles,” in Revue générale de botanique, 25 (1913), 225–252, 285–307, 341–370; most appear in Oeuvres scientifiques (Paris, 1936), which consists of works republished or abridged under the supervision of a group of his students and friends. See also the obituary by Charles Maurain in Comptes rendus hebdomadaires des séances de l’ Académie des sciences, 219 (1944), 144–147.