PERIODICITY IN DEVELOPMENT. I 341 



for weeks, months, or even longer periods, although the capacity for such develop- 

 ment is still retained. These three prominent conditions — rest, activity, death 

 — are in the highest degree characteristic of living things, and to the considera- 

 tion of these and other periodic phenomena we will devote the present lecture. 



This is not the first occasion on which we have encountered such problems, 

 for we have seen elsewhere that under unfavourable external conditions, such as 

 too high or too low temperature, or a withdrawal of water, development comes to 

 a standstill, and finally death ensues. But plants have to endure a scarcity of 

 water sufficient to render development impossible at regular seasons in many 

 parts of the world, and even in our country some plants at least are periodically 

 affected in this way. The same is true of cold, to which our native vegetation is 

 subject every winter. Plants, we shall find, have adapted themselves in a variety 

 of ways, all having for their object the tiding over such unfavourable conditions 

 without suffering permanent injury. We have seen adaptations of this sort 

 already in the behaviour of lichens and mosses to drought ; such forms can 

 remain alive despite a withdrawal of water specifically different in each case but 

 which would at once cause the death of leaves or roots of plants higher than 

 them in the vegetable kingdom. The seeds and spores of these higher forms, 

 however, which, owing to internal causes, are detached from the plant, can 

 endure the withdrawal of water and dry up, remaining alive for a long time even 

 in this air-dry condition, or while containing a very limited amount of water. 

 It is obvious that we cannot describe a seed in the air-dry condition as dead, 

 since it may retain its capacity for germination for several years. The question 

 we have to answer is whether this state of ' rest ' is real and absolute or only 

 apparent. May the seed be compared to a clock wound up and only needing a 

 push of the pendulum to set it agoing, or is the quiescence in the dry seed not 

 really due to abolition of vital processes but only to a diminution of these to 

 such an extent that they cannot be recognized ? The first question that occurs 

 to us is what about respiration, a process which we have seen is essential to every 

 plant activity. Does it cease in dry seeds or only greatly diminish ? This prob- 

 lem has often been tackled, but, according to the latest critical researches, 

 carried out by Kolkwitz (1901), has not as yet been solved. Kolkwitz's experi- 

 ments, which were carried out on barley, show most clearly what an important 

 bearing the arnounjLQf-Wat£i:.piese.nt in the seed has on respiration, sfnce i kg. of 

 barley kept at summer temperatur e gave off in twenty-fouL.hours 3-5gmmg.of 

 carbon -dioxide, when 19-20 per cent, of water was present, 1-4 mmg. with 14-15 

 per cent., 0-3 5 mmg. w ith 10-12 per cent. 



Since there is about 20 per cent, of water in freshly-gathered barley and 

 10-12 per cent, in air-dry seeds one must conclude that respiration decreases 

 very rapidly as dryness is gradually attained, and in air-dry seeds reaches a value 

 which is practically zero, for only about i per cent, of the dry weight of the seed 

 would be respired in 100 years (compare p. 192). It is true we can accelerate 

 the excretion of carbon-dioxide in dry seeds by raising the temperature. At 50° C. 

 Kolkwitz obtained 15 mg. of carbon-dioxide from a kilo of barley containing 

 10-12 per cent, of water. Nevertheless, we can scarcely be wrong in concluding 

 from Kolkwitz's experiments that respiration is not essential to the continuance 

 of a vital capacity, since many seeds are not injured so far as their power of 

 germination is concerned by enduring a much more thorough desiccation, in 

 which cases respiration must be reduced to an amount which cannot be estimated, 

 and thus can have no physiological significance. It is possible to reduce the 

 amount of water in barley to 3, 2, or even i per cent., and Schroder (1886) has 

 shown that barley containing only 2 per cent, of water germinated quite well after 

 an interval of eleven or twelve weeks. No generalizations can be made from these 

 results, and it is probable that fresh experiments may acquaint us with seeds 

 whose power of germination ceases with the stoppage of respiration. 



