32 



THE PLANT WORLD. 



In succulent stems and petioles the arrangement is usually such 

 that the masses in the cortex are tangential, as also in the outer 

 pith, while in the woody ring they are radial and mostly in the 

 pith-rays. The vascular bundles are in this way more or less 

 completely surrounded by ice. 



In leaves, the writer's own observations show that the ice crys- 

 tals first line the spaces of the spongy-parenchyma, but later as 

 freezing continues they completely fill these spaces, or in some 

 cases of leaves rich in water, thev may fuse into a sheet of ice com- 



A B 



Fig. 5. A, section of bud of Lombardy poplar. Note layers of ice in 

 organs. B, section of bud of lilac, white layers composed of ice. Temp. 

 — 10° F. 



pletely separating the upper layers of the leaf from the lower. 

 In bud-scales this latter condition normally occurs in cold weather 

 except in a few species where the scales contain little water. Fig. 

 5 shows the distribution of ice in buds of lilac and Lombardy 

 poplar. The light layers represent the ice. These photographs 

 were taken from free-hand sections with a photo-micrographic 

 apparatus set up out in the open at a temperature of — 10° F. 

 Under these strenuous conditions the negatives were not of the 

 best. 



In twigs there is frequently an ice c>linder entirely around 



