176 PLANT WORLD. 



ing as flocculent granules, may be observed in the cortex. 

 During the period of growth, then, and for some time there- 

 after, the tissues of newly formed stems are practically 

 devoid of rubber. This condition, however, is not con- 

 fined to the new stems. Older stems have, during the 

 same time, been adciing secondary tissues, and these too are 

 devoid of rubber. The outer parts of the medullary rays 

 contain none, nor do the new cortical tissues. The zone of 

 rubber-free tissue shows clearly under a low magnification. 

 Precisely the same physiological relations are shown by 

 plants which have been grown rapidly with abundance of 

 water, except that quantitatively they are much more marked. 

 If, however, plants be grown slowly, even with all the water 

 it is possible to give them, rubber appears, though not in 

 such quantities as occur in field plants subjected to drought, 

 at least In the time that they have been under observation. 

 In addition to the physiological relation, however, there are 

 other changes which take place In Irrigated plants made 

 evident In the anatomical relations. The wood is much more 

 rigid, and this stands in relation to the thickness and depth 

 of the medullary rays, which are much greater in field plants, 

 which also have a thicker cortex, and larger and more numer- 

 ous resin canals. Mutatis mutandis, the wood of an irrigat- 

 ed plant Is three times more rigid than that of a field plant 

 when fresh and freed from the bark ( 1 1 :3.5) and this ratio 

 Is changed to 2:1 after drying. In stems of equal total 

 diameter, the ratio of wood to bark is 6.25 :2.i5 in irrigated, 

 and 5.5 :2.9 In field plants, radial measurements In mm., 

 while In stems with bark of equal thickness the ratios are 

 6.25:2.15 (irrigated) and 2.75:2.15 (field). Without 

 entering further Into details which await fuller publication, 

 It may be said that the changes observed are quite remarkable 

 and have an Important economic bearing. It may be observed, 

 however, that growth precedes rubber secretion, and the 

 more rapid the growth, the longer It takes for the laying 

 down of rubber to catch up with it. Water In abundance 



