172 DIFFERENCE OF ORGANS AT DEVELOPMENT STAGES 



sided pyramidal apical cell characteristic of the stem of the moss. It is 

 true that any cells of the shoot-axis or of the leaves may at a later 

 stage form in regeneration protonema-threads, but not so the vegetative 

 point *. In one instance, namely Schistostega osmundacea, the apical cell 

 of ' enfeebled ' shoots grew out into protonema and therefore it has been 

 assumed that the capacity for this exists in a latent condition in the 

 vegetative point. 



The following examples from the higher plants may be cited. 

 On page 164, I have referred to the band-like primary leaves of 

 aquatic and marsh monocotyledonous plants. It appears that a reversion 

 to the primary leaves may take place in plants whose vegetation is 

 unfavourably influenced (Fig. 104). Plants of Eichhornia azurea, which 

 had wintered as land-plants, produced reversion-shoots with band-like 

 primary leaves -, and the same occurred in Hydrocleis Humboldti, Potamo- 



geton natans, and others. The genus Sagit- 

 taria is specially instructive. The duration of 

 its juvenile form is, as has been said 3 , very 

 ^ variable. It is longest in those species which 

 live more as water-plants, shortest in those 

 which are more land-plants. Amongst the 

 latter is Sagittaria cordifolia in which a reversion 

 could not be brought about ; the plants when 

 cultivated in water usually died off. On the 

 other hand Sagittaria natans, which lives almost 



FIG. 104. Heterantliera reniloimis. Submerged, pOSSCSSCS besides its maiiy band- 

 Seedling plant. After producing reni- i r n i 



form stalked leaves it again produces like submerged leaves only a tew floating leaves, 



band-like leaves as it did at first. ./. ,. n 



whilst in S. cordifolia and others the stalked 



leaves are not floating leaves. Change of the medium and any other 

 limiting cause produced a reversion to the primary leaves in examples of 

 S. natans which had already developed stalked-leaves. 



In dicotyledonous plants reversions have been brought about in an 

 analogous way, for example, in Acacia verticillata, a species producing 

 phyllodes. The young plant here, as in other species of Acacia, develops 

 bipinnate foliage-leaves which gradually pass over into phyllodes. In 

 young plants which had already produced a large number of phyllodes 

 and which were enfeebled by cultivation in a dry chamber, reversion-shoots 

 with foliage-leaves appeared (Fig. 105). The New Zealand species of 

 Veronica and the myrtaceous Melaleuca micromera which have the 

 xerophilous habit recalling that of the Cupressineae, as described above 4 , 

 produce if cultivated in a moist chamber or under unfavourable conditions 



1 See page 47. 



2 See the figure in my ' Pflanzenbiologische Schilderungen,' ii. p. 287. 



4 See page 167. 



See p. 1 64. 



