3i8 METAMORPHOSIS 



drying induced by a temperature of loo'' to iio° C. without being killed, while 

 many mosses are killed by drying in a desiccator. As in general, so also in 

 plants which live under special conditions of life, special peculiarities are de- 

 veloped, whilst plants whose seeds under normal conditions are never desic- 

 cated, frequently do not possess the power of withstanding drought. 



Long before air-dryness is reached turgidity is destroyed, making itself 

 evident by the wilting of the plant members. The capacity to endure wilting is 

 also very varied among plants. Certain succulents can endure a loss of as much 

 as 90 per cent, of the water they contain without suffering permanent ill effects, 

 other plants again can only stand the loss of half the water they normally hold. 

 When turgor is destroyed, growth over all comes to an end. The loss of water 

 may be due either to transpiration under conditions when the supply of water 

 is insufficient, or to osmotic action due to the presence of salt solutions which 

 themselves produce no chemical effect. The results are not identical in the two 

 cases, and this is quite intelligible for the reason that when water is withdrawn 

 osmotically, as has been already pointed out, by entry of the salt or by renewal 

 of the osmotically active material, a reaction follows such as is not possible in 

 a wilted plant. Further, it is difficult for a wilted plant to retain definitely 

 a reduced quantity of water ; it will either re-absorb water and recover or give 

 off more and die. Yet there are many plants which may be preserved in the plas- 

 molysed condition for a long time without death taking place. Algae, for 

 example, may be so treated and preserved alive for many weeks. No growth, 

 however, is observable in them, though new cell-walls may be formed ; every 

 plasmolysed cell in the long run dies. Each alteration in concentration or of the 

 osmotic pressure of the external medium induces injury, and even the periodic 

 changes which take place, for example, in the water in the estuary of a river, de- 

 pendent on ebb and flow of tides, are such as few Algae can tolerate (Oltmanns, 

 1891). 



It will be seen from these remarks that the rate of growth and the final 

 size attained by every plant depend on the amount of water it contains, and this 

 in turn depends on the relation subsisting between absorption from the soil and 

 transpiration into the air. In addition to many other factors the amount of 

 moisture in the air, as well as the quantity of water and salts in the soil, plays an 

 important part. The minimum, optimum, and maximum for different plants, 

 and even for individual organs of a single plant, vary greatly. The researches 

 of Tucker and Seelhorst (1898) on the influence of water on the relationship 

 between the roots and sub-aerial organs of the oat are of special interest. A 

 limited amount of water in the soil excites active growth in the root ; it cannot, 

 however, in spite of its great extent of absorbent surface, supply the aerial parts 

 with sufficient water, and hence these remain small, the relation between the root 

 and the whole plant being i : 7-4 while it reaches i : i6-6 when there is plenty of 

 water in the soil. In this latter case the root remains small, its optimum being 

 already exceeded. Thus between the root and the shoot there are appropriate 

 correlations. 



We may now turn to the effect of moist and dry air as factors in deter- 

 mining form in the plant. It is impossible for us to enter into detail on this 

 question — a statement of the general results must suffice, because the material 

 for discussion is so abundant. It has been shown that retardation and accelera- 

 tion of transpiration very often form a self-regulating mechanism ; the plant 

 in dry air develops adaptations for reducing transpiration, in moist air for 

 accelerating it. The variability of the plant, and especially of the higher plant, 

 has been shown to be far greater in this respect than any one would have 

 imagined twenty years ago. These adaptations are seen not only in external 

 form, but also in anatomical structure. Plants grown in damp atmospheres 

 have longer internodes and petioles, and larger but thinner laminae. The 

 weakly transpiring leaves of Tropaeoliim are, according to Kohl (1886), five times 



