72 CHANGES OF COMPOSITION [chap. 



resemble swedes in composition, but are more watery ; 

 while potatoes contain a much higher proportion of dry 

 matter, about 25 per cent, and store their carbohydrates 

 in the form of starch. When a potato set is planted in 

 spring the starch is converted by a diastatic ferment 

 into sugar, which is moved off to the growing shoots, 

 where it is used for constructing the new tissues ; as 

 growth progresses the cells will be found to be progres- 

 sively depleted of starch until little more than an empty 

 husk is left. A slow conversion of starch into sugar is 

 always going on during the storage of the potato, the 

 sugar being burnt up in the respiration process which 

 keeps the plant alive though resting. It is well known 

 that a potato tastes sweet if it has been frosted or 

 exposed for some time to a low temperature; this is 

 because under such conditions the respiration process 

 is almost suspended, while the enzyme action forming 

 sugar still goes on until there is a perceptible accumula- 

 tion of sugar in the root. 



Among perennial plants the movements of the 

 manufactured materials go on in exactly the same 

 way as in annuals or biennials ; these movements 

 are, however, not so apparent, because of the great 

 permanent store of food material which exists in a 

 perennial plant However, it is instructive to cut sec- 

 tions of the branches of trees at different times of the 

 year and test them with iodine to show the presence of 

 starch : in the autumn the wood, or rather that part of 

 it which is still alive and active, is packed with starch, 

 but this starch is moved out with the first growth of the 

 leaves. It is due to the material thus stored that 

 cuttings can be struck from shoots cut off in the autumn ; 

 they then contain easily available reserve material out 

 of which the young roots and the new leaves can be 

 manufactured until the cutting attains an independent 



