This paper is concerned with the opening and closing processes in the flowers of Crocus and Tulipa. As is well known from the work of Pfeffer, the flowers of Crocus and Tulipa are only slightly photonastic. They are, however, extremely thermonastic. As regards this thermonastic response Crocus is much more sensitive than Tulipa. In the case of Crocus, the temperature may be so low that the flowers cannot open, yet the flowers may develop fully, nevertheless. Thus Pfeffer found that in temperatures below 8° C. the flowers developed but did not open. He found that the flowers of some Crocus plants opened to an extent with fluctuations of only 0.5° C. He made this thermonastic movement more evident by means of a fine silver wire which projected beyond the flower division and which was fastened to the latter by means of lacquer. Others have found that a very fine glass capillary will serve this purpose equally well. My experiments show that the flowers of one species of Crocus are capable of responding to a somewhat smaller temperature change than 0.5° C. This is the case with the flowers of Crocus vernus which has been found to