CERTAIN GENERAL CHARACTERISTICS 



33 



a new thallus. Isolated pieces of the thallus reconstitute with interesting 

 evidences of gradient pattern. Any cell of the thallus is capable of recon- 

 stituting a new thallus, but in multicellular pieces preferential localization 

 of reconstitution occurs (Vochting, 1885). When an anterior region is re- 

 moved by transverse section at any level, a new apical cell and a new thal- 

 lus develop from the ventral side of the midrib just posterior to the cut 

 surface (Fig. 11, B). In general, in pieces containing any part of the mid- 

 rib the new thallus develops from its anterior ventral region (Fig. 11, C, 

 D). In pieces entirely lateral to the midrib with transverse anterior cut 



C E 



Fig. II, A-E. Reconstitution in the liverwort Marchantia. A, unbranched thallus with 

 midrib indicated; B, reconstitution from ventral side of midrib after transverse section; C, D, 

 reconstitution from half-midrib in pieces of half-thallus width; E, reconstitution from most 

 anterior level of piece without midrib and with anterior oblique cut surface (from Vochting, 

 188s). 



end, the new thallus develops from the most nearly median part of the 

 anterior region; but in pieces lateral to the midrib with oblique anterior 

 section, development is from the most anterior region of the piece, which 

 is at the lateral border (Fig. 11, E). These and various other experiments 

 indicate the existence in the thallus of a longitudinal and mediolateral 

 gradient pattern of some sort. Any level of either can be experimentally 

 determined as the locus of development by altering the shape of the piece 

 and the region included in it. Planarian pieces of dilTerent shape and in- 

 cluding different regions give results which are similar in certain respects 

 as regards the gradient pattern which they indicate (pp. 52, 364-65). 

 In multiaxiate seed plants the chief growing tip may completely in- 



