649 



they looked swollen ; in the places where the swelling of the little stems was 

 most intensive, gaping, usually horizontal wounds were found which ex- 

 tended to the pith. In contrast to normal plants of the same age, those of 

 the over-heated soil were only half as long but approximately three to four 

 times as thick in diameter at the place of the greatest swelling. Here too 

 the epidermal cells were two to three times as broad as in normal plants. 

 The stomata showed the same difference only to a slighter degree. The 

 hairs were not changed. The bark parenchyma was, to be sure, four times 

 as thick but no cell increase had taken place; the cells of the pith paren- 

 chyma showed still greater radial distention ; but actual cell increase could 

 be proved only in the bast parenchyma. Prillieux cites further that the 

 nuclei behave similarly. They hypertrophy and increase in such a way that 

 even three or four may be found in a single cell. Nuclear division takes 

 place by fragmentation. Such a cell increase is perceived also in the short, 

 curved and twisted, but not swollen, roots of the changed plants. The large, 

 deformed nuclei show usually very irregular nucleoli, occurring more than 

 one in a cell, in which, not infrequently, vacuoles appear when colored black 

 with osmic acid. In fragmentation of the nuclei, first a fold usually ap- 

 pears at one side and seems to constrict the nucleus. Later a cytoplasmic 

 wall is formed between the two resulting nuclei. The two halves, thus 

 produced, become inflated and tend to separate, which separation, however, 

 does not always actually become complete. It also seems that this cleavage 

 of the nucleus takes place within an already existing cytoplasmic covering, 

 belonging to the original nucleus, which does not rupture until later. 



This increase of the nuclei and the tender bast element may indeed 

 indicate the way in which a higher soil temperature, which approximates 

 the optimum, can act favorably. Cell increase and the conducting of the 

 plastic material may be hastened. As is well known, horticulture makes 

 good use of the beneficial influence of the higher soil temperature by means 

 of hotbeds. Yet just here the observation may be made, that a too high soil 

 temperature is not favorable for the many plants from a cooler climate. 

 They do not grow more rapidly but easily decay. The assimilatory energy 

 slackens and the weakened organism is attacked by bacteria and fungi. 

 Hellriegel's experiments^ show how much assimilation falls when the soil 

 temperature becomes too high. Comparative cultures in roasted quartz 

 sand gave yields for 



rye 

 at 8° 10° 15° 20° 25° 30° 40° C, constant soil temperature 



Fresh weight 191.5 176.3 269.4 456.6 376.0 408.0 240.1 



Dry substance . . 23.9 22.8 32.4 49.5 42.4 47.0 31.2 



wheat 



Fresh weight 98.6 130.8 241.0 260.5 342.0 402.2 296.0 



Dry substance . . 15.8 20.8 29.5 30.8 43.9 46.9 40.3 



barley 

 Fresh weight .... 151.9 156.0 383.4 408.5 435.2 365.,0 230.5 

 Dry substance . . 17.1 18.0 34.4 36.7 42..0 35.0 26.3 



1 Beitr. zu den naturwissenschaftlichen Grundlagen des Aclterbaues. Braun- 

 schweig 1883. Vieweg & Sohn. 



