64 COMPENDIUM OF GENERAL BOTANY. 



of the weight of the leaves and stems, of the snow, ice, etc. The 

 roots of a tree through whose crown the wind blows, and the grass- 

 stem the panicle of which offers resistance to air currents, are subject 

 to & pulling tension. The margins of flat leaves waving in the wind 

 are subject to tearing and breaking. Parts of winding stems 

 wound about dead supports, and more especially those wound about 

 living supports (tree-stems growing in thickness), and tendrils 

 must resist pulling tensions; likewise water-plants in rapidly flow- 

 ing water, and stems of hanging fruit. Rarely there comes into 

 play a supporting tension similar to that of a pillar, as in the case of 

 supporting roots. 1 



The question now is, How are such mechanical requirements to 

 be interpreted ? One difficulty will be to explain these interesting 

 relations briefly, yet not at the expense of clearness. In many 

 respects the brief suggestions given in these lines, in other cases a 

 hasty outlining, will assist in finding the necessary explanations. 



While I shall attempt to demonstrate the mechanical principles 

 in the internal structure of plants by giving a few examples, I shall 

 base my discussion of the subject upon SCHWENDENER'S " Mechanical 

 Principles, etc.," as well as upon NAGELI and SCHWENDENER'S 

 " Microscope." 



As has been demonstrated (SCHWENDENER), there is in the 

 vegetable kingdom a specific mechanical tissue- system, consisting of 

 specific mechanical cells, which in its best quality has the same sup- 

 porting power as malleable iron wire, namely, twenty kilos per 

 square millimeter (within the limit of elasticity}. These mechanical 

 cells are designated by different authors as : stereids, skeleton-cells, 

 mechanical cells, thick-walled bast, hard bast, prosenchyma-fibres, 

 bast-cells, sclerenchyma-fibres. 3 In organs subject to bending the 

 mechanical cells are peripherally located, while in organs subject to 

 a pulling tension they are centrally located ; that is, in typical 

 cases they are arranged according to rational mechanical prin- 

 ciples. That such an arrangement of mechanical cells is a rational 

 one is made clear by the following elementary considerations (com- 

 pare the accompanying figures, 38-42, as well as those pertaining 

 to the root anatomy). 



1 Aerial roots of Zea Mays afford a typical example. It does not seem clear 

 why all vertical tissues are not subject to such a tension. TRANS. 



2 No doubt we must wait some time before a uniform terminology will be 

 adopted. 



