102 PLANT LIFE 
thick and lignified walls, whether they are 
specifically mechanical, or are discharging 
other functions as well, such as that of 
storing supplies of food in the trunk. An 
enormous proportion of the wood of ordinary 
trees consists, then, of thick-walled cells of 
various kinds which are more or less intimately 
knit together, with the result that the whole 
possesses not only considerable strength but 
also a high degree of resilience. This latter 
quality differs greatly in different timbers, 
but it is entirely the result of the properties 
of the individual cell walls, combined with the 
manner in which the cells themselves have 
interdigitated with one another. 
An ordinary tree, by virtue of these pro- 
perties of the wood, is able to withstand the 
effects of a direct crushing stress far greater 
than it will ever be called to meet in nature. 
It has also, by virtue of its resiliency, the 
faculty of recovering its position when it is 
swayed or bent by the wind. 
As regards the great lateral branches of 
large trees, their heavy weight of foliage and 
small branchlets renders the need for power 
of resistence and recovery from strains of 
various sorts even more pressing. Some- 
times, indeed, they prove inadequate, as when 
a branch becomes overloaded with fruit. In 
the ‘present year (1912) the great weight of 
beech ‘mast is causing many large branches 
to bend down till they have come to rest 
upon tlie ground, and in not a few instances 
