TISSUES AND SIMPLE ORGANS 69 



I have attempted to explain in a few sentences with the aid of 

 the figures the mechanical principles involved in the anatomical 

 *trnctnre of plants, yet careful consideration will show that the 

 magnitude of the entire mechanical arrangement of plants may he 

 measured thereby. Countless millions of plant-organs are subor- 

 dinated to the above-mentioned principles. It must also be remem- 

 bered that in the discussion of the vascular system we must again 

 and again recur to this subject. Some related phenomena are yet 

 to receive special consideration. 



Decrease in the Firmness of Flexible Organs iii an Acropetal 

 Direction. — It would be erroneous to conclude that monocotyledons, 

 in distinction to dicotyledons and conifers, were equally thick above 

 and below, since they, as a rule, have no secondary growth in thick- 

 ness by means of a cambium ring. The study of a grass-stem will 

 show this. In the majority of monocotyledons the rejuvenescence 

 upward of the monocotyledonous stem is to be ascribed to different 

 causes from those producing rejuvenescence in the stem of dicoty- 

 ledons. If the expression " becoming thinner above " were changed 

 to " becoming thicker below," we would find that it would be more 

 applicable to dicotyledons than to monocotyledons. It suggests 

 that 2ivao\\g monocotyledons the apical area (vegetative point) has 

 already become somewhat '' firm " before any considerable growth 

 in length takes place. The development of the stem of a palm is 

 quite different from the development of the stem of a dicotyledo- 

 nous tree, although rejuvenescence proceeds upward.' This rejuve- 

 nescence, and especially the weakening of the mechanical system 

 toward the apex, is of great importance. That this is desirable 

 can readily be explained from a mechanical standpoint. A hori- 

 zontal beam of equal thickness throughout and fastened at one 

 end will hreah at the 2)oint of attachnent if too great a weight is 

 brought to bear upon the free end. The point of attachment is 

 first to give way, since there the power arm is longest. If a girder 

 is to have no weakest point, there must be a gradual increase in 

 firmness (to be determined mathematically) toward the fulcrum or 

 point of attachment. In its perfect form we speak of a " girder 

 of equal resistance." Rodenstein ^ at Schwendener's suggestions 



' Eicbler, Growth iu Tbickness of the Stems of Palms, Sitzungsber. der Ber- 

 liner Akademie, 1886. 



•Structure and Life of Plants, III Vereiuscbrift der " Gorres-Gesellschaf t " 

 fur 1879. 



