204 SOME RECENT RESEARCHES IN PLANT PHYSIOLOGY 



stability. Hence, when the wood is rich in water the 

 most rapid transit upwards will be effected in the vessels, 

 which at the same time will be comparatively rich in 

 carbohydrates, while a slower and more dilute stream will 

 pass up in the tracheids. If drought, by causing an 

 increase in tension, produces bubbles in the conducting 

 channels, the major part of the stream, with its solutes, 

 will have to pass up through those tracheids in which no 

 ruptures (bubbles) have developed, thus securing a stable, 

 if slow, supply to the leaves. According to Strasburger's 

 observations (1891), during transpiration the majority of 

 the vessels of the spring wood in the conducting zone are 

 without bubbles. 



The recognition of the glandular function of the wood 

 parenchyma, the translocatory activity of the medullary 

 rays, and the transmission in the tracheae of the circulating 

 carbohydrates, affords a satisfactory explanation of the 

 presence of living elements among the otherwise lifeless 

 tissue of the wood. 



THE MECHANISM OF THE SECRETION OF THE WOOD 

 PARENCHYMA CELLS. 



As mentioned in previous chapters, there is a fall in 

 osmotic pressure as one proceeds from the leaves to 

 the roots. It has been shown by Dixon and Atkins 

 (1915, 2), that the interpretation of this appears to be 

 that there is, during the period of active assimilation, a 

 progressive decrease in the sugars in the direction named. 

 These substances, accordingly, either diffuse from regions 

 of high to those of lower concentration, or their motion in 

 this direction is facilitated by the cells of the bast. That 

 such a motion need not always be downwards is indicated 

 by results obtained by Dixon and Atkins (1910), in which 

 the concentration of sugars in leaves remained very large 



