PRELIMINARY DEFINITIONS. 9 



( i ) Interception of the sun's rays for a longer or shorter period daring the day. 

 The result is that in a warm climate and during the prevalence of warm weather, if 

 the trees are in leaf, more or less protection is afforded to the overtopped plants 

 against excessive insolation and to the soil against loss of moisture and overheating; 

 while in a cold climate the period of vegetative activity each day as well as during 

 the whole year is more or less shortened. The beneficent influence of cover in the 

 former case, however, disappears to a greater or less extent if the trees are out of leaf 

 and even if they are in leaf, provided they are long-boled and standing solitary, for the 

 oblique rays of the sun, after striking on the bole and on the bare branches, if any, are 

 then reflected downwards and add'enormously to the effects of direct insolation. Ther- 

 mometric observations in the Dehra Dun forests have proved that when the trees are 

 out of leaf, the shade maximum is higher under the trees than out in the open. A 

 second result of the interception of the sun's rays is that assimilation is checked or 

 at least retarded, but this effect is the slighter, the higher the crowns of the trees are 

 above the ground. 



(ii) Impeded radiation. This delays and diminishes the deposition of dew; but 

 on the other hand, the deposited dew, if any, is evaporated more slowly, and frosts, 

 particularly night-frosts, are mitigated and may even be entirely prevented. Never- 

 theless, where heavy dews occur, the soil under the crowns receives a large quantity 

 of moisture every night and morning by means of the drip from the leaves. 



(iii) Rain prevented from falling directly. This is beneficial for the preservation 

 of the soil on steep slopes, but it acts unfavourably on the overtopped plants, which 

 require the direct impact and even distribution of the rain-drops to wash off the dust 

 and other extraneous matters that cover and clog the surfaces of the leaves. Moreover 

 the heavy drip from the crowns, falling constantly at a few points, may not only shake 

 severely and thereby injure young seedlings, but also scoop out hollows in the soil 

 and expose their roots. It must also be noted that a portion of the rain 

 caught by the crowns never reaches the soil but is evaporated back into the atmos- 

 phere ; but, on the other hand, a very much larger proportion of the rain that falla 

 is retained by the soil to benefit ultimately the roots ofthe trees. Moreover the soil 

 is protected from the direct impact of the rain and from heavy surface drainage. 



( iv ) Protection from hail. 



(v) Exclusion or at least tempering of violent or h^t or cold winds. An indirect 

 effect of this action is to enrich the soil by preventing the covering of dead leaves 

 from being blown away. All the effects just enumerated are very much more 

 marked in the case of canopied forest than in that of isolated trees. 



From what precedes it is evident that shade is not synonomous with cover, since it 

 is only one of the effects of cover and moreover moves with the sun, whereas cover 

 is always stationary. Similarly the terms cover and shelter must not be confounded 

 together, for shelter is partially an effect of cover and sums up all the desirable eflects 

 of cover. Moreover it extends far beyond the area of projection of the crowns of the 

 trees. 



63. A plant is said to be SUPPRESSED or UNDER SUPPRESSION 

 when its growth has been or is being injured by cover, and, con- 

 versely, one plant is said TO SUPPRESS another when it affects in- 

 juriously the growth of the latter by its cover. 



64. A NURSE is a tree or shrub intended to protect against in- 

 jurious weather influences young plants until they are sufficiently 

 established to no longer require such protection. Hence TO NURSE 

 signifies to perform the office of a nurse. 



65. DORMANT or LATENT buds are those buds which, without 

 losing the power of sprouting, remain quiescent and burst forth 

 into leaf only under specially stimulating conditions, as when, for 

 instance, owing to the natural or artificial curtailing of the existing 

 foliage, there is a determination of the developmental energy of the 

 plant to those buds. 



