STEMS 727 



Moisture as a factor in elongation. — Certain of the Crassulaceae 

 (notably Sempervivum assimile, fig. 1043) usually are stemless in their 

 natural xerophytic habitats, while growth in moist, light chambers results 

 in the conspicuous development of erect leafy stems (fig. 1044); growth 

 in moist, dark chambers results similarly, except that leaf formation is 

 greatly reduced (fig. 1045). These and many similar experiments show 

 conclusively that moisture favors elongation, while desiccation results in 

 shortened and often in laterally enlarged stems; as might be expected, 



Fig. 1046. — A potato plant (Solanum tuberosum), grown from a tuber planted with 

 one end in the soil, but with the larger portion in the air; note the short and stout aerial 

 shoots, which contrast strikingly with the long and slender shoot which originated in the 

 soil; note also the much larger leaves on the latter shoot. — Photograph by Fuller. 



the presence of soluble salts in the culture media produces the same effect 

 as desiccation. There are few if any similarly conclusive experiments 

 as to light, and it seems probable that moisture differences constitute the 

 chief factors in determining the phenomena noted in the preceding para- 

 graphs. Indeed, in some cases (as in Jussiaea and in the seedlings of 

 Vicia Faba) there is greater elongation in illuminated than in dark cul- 

 tures, if the moisture conditions are equal and favorable. Stem inter- 

 nodes become shorter and thicker and the plant more bushy, if the ordi- 

 nary supply of carbon dioxid is doubled or quadrupled, but in nature this 

 factor is not likely to be important. Variations in the length of petioles, 



