SECTION II. 



INTURIOUS ATMOSPPIEKIC IXFLUKXCKS. 



CHAPTER IV. 



TOO DRY AIR. 



Injury to Buds. 



Although in house plants, for example, we have constantly met with the 

 lack of sufficient atmospheric moisture as a factor in the production of the 

 phenomena of disease, it has as yet been but very little taken into 

 consideration. 



The direction in which continued great scarcity of atmospheric moisture 

 makes itself felt may be seen from the peculiarities of the xerophytes. As 

 an example of this, we will mention Grevillius'. He found in the plants 

 of a treeless lime plateau a thickening of the epidermis and its wax coating, 

 or, as a substitute for this, a great increase of pubescence. These char- 

 acteristics are more marked in leaves near the top of the stem. The e[)i- 

 dermal cells, in contrast to normal forms, usually have somewhat smaller 

 lumina. The palisade cells are broader and more closely joined to one 

 another, the intercellular spaces are smaller; the mechanical tissues in the 

 branches and petioles are better developed, the pith less ; it has smaller cells 

 but is richer in starch. These changes, in fact, occur almost always in con- 

 nection with a great lack of moisture in the soil whereby it is hard to judge 

 which is due to the dryness of the air alone and which to the excessive 

 transpiration conditioned by it. However, we find various processes setting 

 in when, with a sufficient supply of soil moisture, the air is constantly hot 

 and dry ; these will have to be discussed here. They are in part phenomena 

 of arrestment in the life of the buds or in the conditions of germination; 

 in part disturbances in the mature leaves which lead to the falling of the 

 leaves in summer. 



Two stages must be noticed in the life of the buds and the develoi^ment 

 of the young shoot after the bud has unfolded. If a considerable dry period 



1 Grevillius, Morphologisch-anatomische Studien lib. d. xerophile Phanero- 

 gramen -Vegetation der Insel Oeland. Englers Jahrbiicher 1897, XXIII, p. 24. 



