282 EXPERIMENTS WITH PLANTS 



When the untreated cuttings have put forth vigor- 

 ous shoots, those with vaseline on them will probably 

 show little or no development of shoots. Now care- 

 fully wipe off the vaseline with a rag, and then rub 

 the stems well with dry sand and place them again in 

 the chimney or set them in water, to see if they 

 will grow. 



Cuttings and seeds sent long distances, especially 

 to or from the tropics, are frequently ruined by start- 

 ing to grow prematurely in transit. Of the three fac- 

 tors which promote growth, namely, moisture, warmth 

 and air- supply, the attempt is constantly made to 

 control the first two, which is in most cases difficult 

 or impossible, while the control of the air -supply, 

 although easily accomplished, has been neglected. 

 Sealing up the plants in air-tight tins or packages 

 has been tried, but this is far less effective than seal- 

 ing each cutting by means of wax, paraffin or similar 

 substances. This seems to promise a favorable field 

 for experiment. 



How do the submerged portions of water-plants 

 get air ? When they have leaves which project above 

 the water or float on its surface, they may receive a 

 supply of air through them. Examine some plants of 

 this sort (e. g., Pond Lilies, Arrowhead, etc.), and 

 notice what enormous air -passages exist in them. Do 

 these passages extend up into the leaves? Examine 

 also swamp plants which have their roots in water or 



