FALL OF THE LEAF. 355 



superfluous, and usually fall off — or, if they still remain on the lateral veins, they 

 are shrivelled, insignificant, and meaningless. 



The dry membraneous scales seen on young fem-leaves should be mentioned 

 here. Let us examine a frond of the first wild fern we meet — say of Nephrodium 

 Filix-mas. The young frond is still spirally rolled, although it has forced its way 

 through the soil, and is now exposed to the wind. Moreover, nothing is to be seen 

 of the fresh green which later adorns this fern; the lower part of the midrib and 

 lateral veins appear to be stre^vn with chaff", being entirely covered with dry 

 membraneous brown scales and shreds. Later, as the leaf unrolls more and more, 

 its green fronds also become expanded, but by this time the cell-walls are sufiiciently 

 strengthened, and no longer require the chaffy coat. In ferns which grow in 

 sunny, rocky situations, and as epiphytes on the fissured bark of old trees in 

 tropical regions, this coat of chaflfy scales is even more noticeable, and, as stated 

 earlier, in such plants it persists throughout life. 



FALL OF THE LEAF. 



Just as many phenomena of the sprouting and unfolding of foliage are dependent 

 upon transpiration at the beginning of the vegetative period, so many processes, 

 but chiefly that of the fall of the leaf, stand in causal connection with transpiration 

 at the close of that pei-iod. Sooner or later, of course, the activity of each leaf 

 entirely ceases; it dies, becomes detached from the stem to which it has rendered 

 service, and falls to the gi-ound, where it decays. In districts where the vegetation 

 can continue its activity uninterruptedly throughout the year, there is nothing verj' 

 noticeable about the fall of the leaf. As a rule, as the new leaves arise below the 

 growing apex of the shoot, the lower, older leaves wither up and decay; the fall 

 is quite gradual, and takes place, like the development of new leaves, all through 

 the year. In neighbourhoods, however, where the changes of climate prevent the 

 uninterrupted activity of plants throughout the year, it is essentially different. 

 Trees and shrubs, and many smaller plants, shed the whole of their foliage in a few 

 days at certain annually-recurring periods, and then remain with bare branches for 

 a considerable time, apparently quite lifeless. This is the case in regions where a 

 long, hot, dry period follows the short rainy season, and also in very cold districts 

 where the long-continued frost causes an icy winter, and the plants are locked in a 

 deep sleep. In tropical and sub-tropical regions, where no showers occur for many 

 months at a time, the branches become stripped of their leaves. Even at the begin- 

 ning of the dry hot season, they remain apparently dead for months, but again 

 break out into leaf at the commencement of the cooler rainy season, when invigor- 

 ating moistui-e is supplied to the parched ground. On the other hand, in those 

 regions of the temperate zone in which there is no sharp distinction between the 

 rainy and dry seasons, and rain falls every month, the foliage is stripped from the 

 trees at the beginning of the cold period, and after the winter is over, fresh gi-een 

 leaves once more burst from the buds on the branches. 



