STEMS 747 



tubeiization, the presence of abundant sugar also is necessary. Since 

 fungi commonly have a much more concentrated cell sap than do other 

 plants (p. 755), it seems very probable that the role of fungi in tuberiza- 

 tion is in raising the concentration of the media which they enter ; in- 

 deed, it has been demonstrated that Fusarium cultures in macerated 

 preparations of potato tubers raise the concentration. Very probably 

 tubeiization resulting from low temperatures or from soil dryness may be 

 similarly explained by increased cell sap concentration. In some cases 

 (as in the radish) the increasing concentration of the sugar manufactured 

 by the leaves may alone be sufficient to stimulate tuberization. 



Stem succulence. As in leaves, so in stems, succulence appears to be of two fun- 

 damentally different sorts, namely, halophytic and similar succulence, in which 

 water accumulation appears to be associated with a concentrated cell sap, and 

 the sort of succulence found in the cacti, which rarely occur in alkaline situations 

 and whose cell sap is not known to be particularly concentrated. In Salicornia 

 and in other fleshy halophytes, succulence has been shown to vary with the salt 

 content of the medium, and the retention of water would appear to be due largely 

 to the presence of salts. In the cacti and in similar forms the retention of water 

 probably is due in large part to a thick cuticle and to other relatively impermeable 

 tissues, supplemented by a small transpiring surface. As stated elsewhere (p. 632), 

 the cells of succulent plants do not necessarily contain more water than do those 

 of other plants, but they are more compactly placed. Axial shortening and dia- 

 metral enlargement are significant features of succulent stems, and in these re- 

 spects such stems agree with tubers and with xerophytic stems generally; 

 furthermore, these features appear to be due to similar causes. 



Correlation ; regeneration ; polarity. Correlation. In the preced- 

 ing chapters much has been said concerning the influence of explicit 

 external factors, such as light, temperature, and water; allusion has been 

 made also to internal or inherent factors, which are supposed to represent 

 hereditary as opposed to environmental influences. Intermediate be- 

 tween these are influences residing within the organism, but not heredi- 

 tary, such as the influence of one cell or organ upon the development of 

 another. Phenomena due to such influences are known as correlations. 

 If an individual of Sempervivum assimile, ordinarily a stemless plant 

 with succulent leaves, is removed from its natural dry habitat to a moist 

 chamber, there soon develops as a reaction to the new conditions an erect 

 stem, on which the first leaves also are succulent (figs. 1043, 1044). After 

 a time thin leaves develop, whereupon the stem ceases to elongate. Here 

 axial elongation is correlated with thick leaves, and axial shortening with 

 thin leaves. Many plants (as Penthorum and Satureja, figs. 981, 985) 



