Germination of the Cocklebur

GERMINATION OF THE COCKLEBUR. BY C. G. VAN BUREN, B. S., OF THE BRIGHAM YOUNG UNIVERSITY. Most farmers know that each cocklebur bears two seeds, and many of them have probably noticed that one of these seeds (the lower one) sprouts, or germinates, the first year, and the other (the upper or the one nearest the apex of the bur) germinates the sec- ond year. This peculiarity has lead to much speculation, and for a long time has puzzled botanists. The most frequent cause of delayed germination is the water-proof character of the seed coats which hinders the entrance of water, and when moisture is excluded germination cannot take place. However, it was shown that both seeds of the cocklebur imbibed about an equal amount of water, therefore that could not account for the difference. At length it was suggested that "enzymes" (ferments which have the peculiar property of changing starch into sugar) were readily produced in the lower seeds, but in the upper ones only after a long period of rest. These enzymes digest the food for the young plants, hence those seeds having them could begin growth immediately, while those seeds without them could begin growth only after one or more years. To test this theory Mr. Wm. Crocker began work in the Hull Botanical Laboratory of the University of Chicago. He obtained only negative results, but accidentally discovered that the seed would germinate at a high temperature; also that some of them, whose coverings were damaged while being removed from the bur, began to germina*-e at lower temperatures. These discoveries led to a new line of experiments which demonstrated some interesting facts. Mr. Crocker found that at 22 to 24° Centigrade (72.6 to 76.2° Fahrenheit) 99 per cent of the lower seeds germinated but

982 IMPROVEMENT ERA. none of the upper ones. At a temperature of 32-34° C. (90.6- 94-2° F.) 100 per cent of the lower seeds and 99 per cent of the upper ones germinated. He then removed the seed coats and found that all the seeds, the upper as well as the lower ones, would germinate at 18-22° C. (65.4-72.6° F.) The lowest temperature at which the lower seeds would germinate with the seed coats on was 21° C. (70.8° F.); the lowest, or minimum temperature for the upper seeds fluctuated between 25° and 41° C. (78-106.8° F.) but both the lower and the upper seeds, with the seed coats removed, germinated at a temperature as low as 18° C. (65.4° F.). This was evidence enough to show that the seed coats and not the enzymes caused the delay in germination. The next thing to do was to find out why the seed coats re- tarded germination. Besides warmth, seed must have moisture and air (oxygen) in order to germinate; evidently, then, the seed coats prevented the entrance of either water or oxygen. Mr. Crocker found that both the upper and lower seeds imbibed about the same amount of water in a given time, so it became evi- dent that the delay was not due to the lack of moisture. By soak- ing the upper seeds and then placing them in a jar of oxygen at a temperature of 21-23° C. (70-8-74.4° F.) all germinated in six days. Further experiments showed that at 23° C. (74.4° F.) the lower seeds would take up 1.6 to 1.7 times as much oxygen with the seed coats removed as with them intact, while the upper seeds took up 2 to 2.4 times as much oxygen with the seed coats removed as with them intact. As a temperature of 32° C. (90.6° F.) is rather unusual in the springtime, and as man, or any other animal, so far as we know, is not particularly concerned in removing the seed coats, the question naturally arises: why should these seeds germinate at all in nature? Mr. Crocker has not forgotten this interesting feature. He took: 1st, a quantity of seed just gathered; 2nd, seed that had been stored in the laboratory one year; and 3rd, seed that had been stored four months in the laboratory and then buried eight| months in the ground. On placing these seeds under germinating conditions he found after a period of thirty days at a temperature of 28-33° C. (83.4-92.4° F.) 3 per cent of the first lot germinated, none of the second lot, but of the seed that had been buried eight months 96

GERMINATION OF THE COCKLEBUR. 983 per cent were growing. He then removed the seed coats from other quantities of seeds taken as above, and in 33 days, at a temperature of 18-22° C, (65.4-72.6° F.) all of the seeds just gathered and those stored in the laboratory one year had germinated, but only 94 per cent of the seed that had been buried for eight months. These experiments show that when buried in the earth the seed coats undergo a change, and permit the passage of oxygen, but the "vitality" of the seed is slightly impaired. Under the microscope the coats of the upper seeds are shown to be a little thicker than those of the lower ones, and the burs around them are harder and do not decay so soon, hence the ability of the upper seeds to delay germination under natural conditions. There no one but can see how admirably the cockleburs are is adapted to cling to the fur of passing animals, and thus secure the distribution of their seeds in space, but even more admirable is this other quality which enables it to distribule its offspring through time. Provo, Utah.