IRRITABILITY 191 



subjected? Of these buffet-ted forms the Sea Palms (Pos- 

 telsia paJm&formis) of the Pacific Coast are the most strik- 

 ing. Living between the tide-marks, always in the most 

 exposed positions, these upright plants hold on and grow in 

 spite of the tremendous pounding to which they are almost 

 continually exposed. In toughness, strength, and elasticity 

 their upper parts are equalled only by the closeness of the 

 attachment and the strength of the hold-fasts. The ,accom- 

 panying figure suggests how rough their habitat may be in 

 a storm. 



From the foregoing we may conclude that a certain 

 amount both of freedom to move and also of actual agita- 

 tion is good for plants. This is exercise, apparently as de- 

 sirable for plants as for animals, and presumably for the 

 same reasons. It facilitates the transfer of nutrient sub- 

 stances and it stimulates the living protoplasm. Which 

 factor is the more important it remains for experiment to 

 determine. 



In trees and shrubs the mechanical tissues are found espe- 

 cially in the wood. Where the seasons are sharply con- 

 trasted, as over the greater part of the temperate zones, the 

 wood presents the familiar appearance known as annual 

 rings. In mechanical strength the different parts of the 

 wood vary considerably, the so-called ''spring- wood," be- 

 cause of the larger size of the cells and the comparative 

 thinness of their walls, being decidedly weaker than the 

 thicker-walled, more compact, and often more abundant 

 ''autumn wood." It is through the wood, especially the 

 ducts and tracheids, that the transfer of food-materials 

 from roots to leaves takes place (see pp. 119-124). The 

 wood is, therefore, both a mechanical and a vascular tissue. 

 The one function or the other predominates at different 

 times during the growing season and affects the growing 

 and developing tissues accordingly. 



Where growth is always possible and is practically con- 

 tinuous, annual rings are not formed. It is only where 

 growth is periodic because of changing seasons, like winter 

 and summer, dry and rainy seasons, that there are decided 

 differences in the character of the wood. Certain other 



