TOPOGRAPHY OF SKELETON 83 



fruits, and leaves, where they occur in more or less isolated 

 groups, they give hardness and toughness without being an 

 impediment to increase in size. 



The stone cells are frequent and important landmarks in the 

 study of powdered drugs and condiments. Fig. 38 shows a 

 variety of forms, enough to give a general conception of their 

 visible characters. 



Topography of the Skeleton.— Purely mechanical consid- 

 erations cannot alone be taken into account by plants in the 

 location of the skeletal tissues. The trunk of a tree, for instance, 

 is something more than a strong column to bear the crown aloft : 

 it is also a part of the body through which water and food must 

 circulate and be stored; respiration, digestion, and assimilation 

 must occur in it as in other members of the plant; and cell division 

 and growth must take place there, and these functions are quite 

 as important as that performed by the skeleton. Therefore 

 the problem before plants in the building of their skeletons is 

 to follow the best mechanical principles wherever this can be 

 done without too great sacrifice of the other functions. 



In Dicotyledons it is of the utmost importance that the parts 

 of the skeleton be so placed that they do not obstruct secondary 

 increase in. thickness; while in Monocotyledons, where increase 

 in thickness continues, as a rule, but for a brief period, this 

 consideration is of much less importance. 



In stems which have to bear the weight of the crown, and 

 withstand the stretching and compressing stresses as they are 

 swayed back and forth by the winds or other agents, the best 

 position for the skeleton tissues from a purely mechanical stand- 

 point is at the outside of all other tissues, in the form of a hollow 

 cylinder; or, other things interfering, as near this form and 

 position as possible. But in all of the higher land plants it is 

 an absolute .necessity to have a tissue at the exterior suited 

 to prevent loss of water, or to keep the water from filling the 

 intercellular air spaces in case of the higher water plants, and 

 so the skeleton must give way to the epidermis and cork. Further, 

 in Dicotyledons, a complete skeletal cylinder at or near the 



