54 LIFE : OUTLINES OF GENERAL BIOLOGY 



again, like thistle-do\NTi aiid dandelion-down, are minute nutlets 

 with a parachute of silken hairs which the wind wafts hither and 

 thither till some of them, at least, are caught on suitable places. 



Very characteristic of autumn is the withering and fall of the 

 leaves. They have worked hard all the summer, but they would be 

 sources of weakness in winter, when soil-water is less available and 

 when freezing of delicate tissues is apt to occur. So the leaves, 

 transfigured in dying, are shed in the autumn, which is often appro- 

 priately called "the fall". It is interesting to find that before they 

 are separated off by a partition which heals the wound they sur- 

 render to the stem almost all that is worth having. The fallen leaves 

 which the earthworms bury contain little more than dead tissue and 

 waste products. 



What the trees do in surrendering vulnerable parts is also seen 

 among animals. Thus many of the plant-like zoophytes in the shore- 

 pools "die down" in the autumn, reminding one of herbaceous plants. 

 The green fresh- water sponge dies away in autumn, all but little 

 pinhead clusters of cells called gemmules, which eventually float 

 away from the dead skeleton and start new sponges in spring. The 

 sacrifice of parts or members finds expression in some of the social 

 insects in a striking way, for of the wasp colony ajid the humble-bee 

 family only the young queens are left to face the winter. In the 

 beehive there is an eerie cold-shouldering and final killing of the 

 drones in autumn. No doubt autumn must be thought of as a time 

 of retrenchment! 



Characteristic of the season and symptomatic are the "flights" 

 of small gossamer spiders, a passive migration on the wings of the 

 wind; and another note is struck in the industry of earthworms, 

 which are now at their busiest in taking withered leaves under- 

 ground. 



Winter is a difficult time for many plants and animals in North 

 Temperate countries. It is a yearly reminiscence of the Ice Ages 

 which have occurred repeatedly in the history of the earth and 

 have from time to time severely pruned the growth of life. Short 

 days, low temperature, stormy weather, scarcity of food — these are 

 sharp pruning-hooks of winter. The problem is to keep alive, and 

 there are many solutions. Neatest of all is the migration solution, 

 for the birds that come as summer visitors to North Temperate 

 countries take their departure in late summer or autumn for more 

 genial climes. Thus, they know no winter in their year; they have 

 not only annihilated distance, they have circumvented the seasons. 

 Very different is the hibernation-solution exhibited by some im- 

 perfectly warm-blooded mammals, like the bats, which seek out 

 secluded nooks where the temperature kee{)s above that outside, and 

 sink into a strange almost reptile-like state. Out of their weakness 

 in being imperfectly warm-blooded, they have made a strength. 



