838 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



this the temperature rises materially above that point, the sap flows. 

 Alternate freezing and thawing, moderately warm days followed by 

 freezing nights, are the ideal meteorological conditions that promote the 

 flow. So long as the air temperatures remain constant, there is little or 

 no sap flow. 



Conclusions as to the Causes and Significance of Sap Pressure and 

 Movement. — If temperature and pressure records are compared during the 

 period of active sap flow, it is clear that although there is a general relation- 

 ship between the two, it is not as constant or exact as would be the case if 

 the cause were purely physical. One of the strongest arguments against 

 reliance on a physical examination is found in the sudden cessation of the 

 sap flow at the end of the sugar season, when there are not only wide 

 temperature variations, but also the highest water-content found in the 

 trunk during the whole year. 



Sachs pointed out another kind of bleeding phenomenon, associated 

 with the activities of living cells, viz. the tendency of certain roots and 

 other living plant tissues to exude sap under certain circumstances 

 independently of corresponding temperature changes. Wieler found that 

 a great variety of plants show active bleeding phenomena, and that this 

 bleeding is a function of living cells, and controlled by their activities. 

 The process is more especially a function of certain cells of the xylem, or 

 woody tissues, and there is a general tendency of these plants to bleed 

 excessively in the early spring. Wieler concludes that this exudation 

 must be due to permanent differences in the character and activities of 

 the protoplasm on the opposite sides of the living cells concerned, possibly 

 associated with local differences in the cell- wall or other factors as con- 

 tributing causes, but that in any case the activities of the living proto- 

 plasm are the dominant factors. 



It seems to the authors of the bulletins that most of the phenomena 

 of sap pressure and flow which are observed in the Sugar Maple can be 

 understood only by such an appeal to the activities of living cells, which 

 activities are, of course, highly sensitive to physical and chemical modifica- 

 tions in the environment of the protoplasm. 



Accordingly, they assume that strong sap pressures and resultant 

 movements are undoubtedly occurring in the untapped Maple tree at the 

 season when the farmer by tapping secures the sap, i.e. the untapped 

 tree "bleeds internally." What is the significance of this in the life of 

 the tree ? Since there seems to be no adequate foundation for the view 

 that bleeding is a mechanical aid to the opening of buds by increased sap 

 pressure, it seems more likely that the object is to secure the fullest 

 injection possible of the plant tissues with water, in preparation for the 

 largo demand for water associated with leaf expansion and excessive 

 transpiration. The especial liability of the Maple to "leaf scorch" or 

 "tip burn" in the hot dry winds of spring indicates that in this tree 

 transpiration evidently exceeds root absorption to a dangerous degree, 

 whenever atmospheric conditions especially accelerate the former. 



Fi A. W. 



Marchantia, Spermatogenesis of. By Professor S. tkeno (Beth. 

 Bot. Cent. xv. pp. 65 88 ; with one plate and one figure in text). — Traces the 



