5S 



others would develop normally. It is a question whether the 

 activity-hypoplasia. of which we v/ill speak later in detail, 

 lets an inactivity hypoplasia he set up in opposition jj it. 

 I have already indicated that cases of this kind are not yet 

 knov/n. The comparison of plants matured under water, which 

 are supported hy the surrounding medium, on which account 

 hut little is required of them mechanically, with individuals 

 from moist cultures on which mechanical demands are made, 

 makes it perhaps impossihle ( see above) to explain the omis- 

 sion of the mechanical tissues in the former as " inactivity 

 hypoplasia" . Besides this, all tissue forms in plants matured 

 under water are weakly developed, just as in specimens grown 

 in jjioisture or in the dark. There is no foundation for the 

 supposition that a reduction of the mechanical tissues would 

 regult from non-utilization. For the same reasons we may 

 no* speak of "inactivity-hypoplasis" v/hen no normal assimil- 



atory tissue is developed in plants grown in cultures in 

 the dark, or in places free from carbon dioxide, from which 

 the opportunity of assimilation was taken away, etc. 



Tschirch explains the weak development of the mech- 

 anical ring in weeping varieties of different trees by the 

 fact that less rigidity is required of their branches than 

 of those of upright forms. Experimental proofs supporting- 

 Tschirch' s supposition do not exist, rather, 'Wiedersheim' s*^ 

 new investigations make it seem impossible that the slight 

 surplus of mechanical requisition to which the branches of • 

 upright forms occasionally the young branching ones, are 

 subjected^ ^could incite the branches of the weeping forms 

 to a stronger formation of their mechanical ring. Therefore, 

 we are not justified in terming this "inactivity-hypoplasia." 



Tissue hypoplasias similar to those expressed in plants 

 by a reduction of the cell size, decrease of the cell number 

 (54) and simplification of the cell and tissue differentiation, 



may doubtless be pointed out in the same diversity i n animal 

 organisms. 



1, Beitr, z. Kenntn. d. mechan. Gewebesystems. Prings- 

 heim's Jahrb. f, wiss, Bot. , 1885, Bd. XVI, p. 529. 



3. Ueb. d.^Einfl. d. Belastung auf d. Ausbildung v. 

 Holz- and Bastkorper bei Trauerbaumen , Ibid., 1902, 

 Bd. XXXVIII, p. 41. 



