156 THE PLAN'T WORI.D 



into some soluble substance — generally sugar. The change which we 

 can bring about by boiling with acids is effected in the plant by means 

 of the so-called "ferments." These are complex chemical compounds 

 that occur in all plants. Perhaps the most familiar one is "diastase," 

 the ferment that is found in malt, and can change starch into malt sugar, 

 glucose, and dextrin, a kind of gum. Another familiar ferment which 

 occurs in the animal body is "pepsin." As the season of growth 

 approaches, the reserve of starch is acted on by these ferments, and the 

 sugar which is formed is carried in the sap to all parts of the plant. 



Nearly every one has noticed that when a vine clings closely to a brick 

 or stone wall the parts near the chimneys often send out leaves before 

 the buds on the rest of the vine have begun to grow. It is probably due 

 to the action of the ferments on the starch in the stems, under the in- 

 fluence of the greater warmth of the wall near the chimney. 



In another way preparation is made for the resting season, for the 

 young wood of perennials is ' ' ripened. ' ' The tissues become harder and 

 less watery, so as to withstand cold weather better. It is not so much the 

 cold that injures plants as it is the harm done by their freezing, which 

 tears apart the tissues by the expanding ice. The less water there is in 

 the tissues, or, in other words, the more concentrated the sap, the less 

 danger there is of freezing, just as the freezing-point of water is lowered 

 by dissolving salt, or sugar, or something else in it. 



Preparations are also made for the fall of the leaves by the formation 

 of a layer of cells across the stem . Certain of the cell walls are destroyed 

 and the leaf then falls off. Sometimes a layer of corky cells is formed 

 across the end and the tubes in the fibrovascular bundles are closed by 

 a sort of gum that prevents bleeding. 



Is it not evident that any attempt of ours to make the plants grow 

 actively through the winter must meet with failure ? All the arrange- 

 ments and habits of the plant are interfered with. The best we can hope 

 for is that the resting period will be shorter than usual, and this is found 

 to be the case. Our own observations have been made chiefly on ferns 

 grown in a glass case indoors. The evergreen species and a few of those 

 that die down in the fall remain green, while the others wither away. 

 This year the new growth of most of them began late in February, nearly 

 or quite two months before the time outdoors. At the time of writing 

 the maidenhair, walking fern, and Cystopteris (two species) are still 

 dormant, while the ebony spleenwort, Bradley's spleenwort, Christmas 

 fern, lip fern i^Cheilanthes) , and others are sending up new fronds. A 

 large hart's tongue is also showing signs of activity, for its fronds are 

 slowly starting to uncoil. In a near-by aquarium some plants of bladder- 

 wort that appeared to be dead have recently started to grow rapidly, 

 vying with the Chara around them. 



