FACTS CONCERNING VARIATION 195 



peratures is only gradually attained after a long series of generations. 

 Furthermore, an external change may be the outcome of both forms of 

 variation, which alike demand an internal change in the protoplast. 



Plants are not all equally capable of variation, whether saltatory or 

 gradual. Nor in asomatophytes can all reactions to changed conditions be 

 permanently fixed by continued culture, and even when this is possible, 

 special conditions may be required. Thus in fermenting yeast the power 

 of spore-formation is never lost, although countless generations may succeed 

 one another without it ever occurring. On the other hand, high temperatures 

 produce an asporogenous variety from non-fermenting yeast. Similarly, if 

 Mucor is continually cultivated in yeast-form by growing it in a nutrient 

 medium covered with a thin layer of oil and frequently reinoculating, it 

 never loses the power of forming sporangia and spores when returned to 

 normal conditions. 



The fact that a variation capable of hereditary fixation gradually 

 disappears if not sufficiently fixed, does not show that in all cases where an 

 after-effect is shown, it may also be rendered hereditary. Thus the after- 

 effects of accommodation to poisons, high temperatures, and concentrated 

 solutions disappear in the course of a few generations, not only in the case 

 of asomatophytes, but also in that of somatic fungi. Similarly the periodic 

 daily movements of leaves gradually cease in continuous darkness, although 

 generation after generation has repeated the same rhythmic response to daily 

 changes in the illumination. In many cases, however, the yearly periodicity 

 has apparently become hereditary and is impressed upon the seeds, as is 

 instanced by the earlier ripening of corn obtained from northern regions. 

 Seedlings germinated in darkness, however, never show the daily leaf- 

 movements which were performed by the parent with every change from 

 day to night. 



The persistence of an after-effect shows that the plant has to overcome 

 a certain internal inertia in accommodating itself to the altered conditions, 

 but in some cases the embryonic cells become rapidly accommodated. 

 A certain plasticity is essential to maintain the inherent character of the 

 species under varying conditions, and is even retained by cells which have 

 undergone considerable specialization but which still remain meristematic. 

 Whenever a new plant is produced by such cells it has the same characters 

 as one produced from typical embryonic cells, but it is difficult to say 

 whether this would be the case if every somatic cell were capable of 

 reproducing a new plant Supposing a pollen-tube could be caused to grow 

 indefinitely, or even reproduce itself, we should have a plant whose peculiar 

 characters were due to some internal change during the process of matura- 

 tion. As a matter of fact, however, although pollen-tubes may attain a con- 

 siderable size on suitable sterile media, death always ultimately supervenes. 



Saltatory variations may be more pronounced than the most marked 



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