GREGARIOTJSNESS AND SOCIABILITY OF SPSCTE3. 97 



youth by the dense and stronger roots of other species, even of the 

 grasses. 



(t) Its winged seed, which in spite of its size and great weight, 

 is often wafted away more than 50 yards from the parent tree. 



(u) Its great resistance to frost, except in damp low localities. 



Si. 



ISSU. 



This tree, as a gregarious species, is confined to the numerous 

 flat expanses of loose silty deposits left by the flood-waters of 

 Himalayan streams, where, after debouching into the plains, they 

 spread out over a great width, often branching into a number of 

 shallow arms, which embrace between them islands of various sizes. 

 The loose open soil, from being for a time under water, occasionally 

 dries down to a depth of as much as twenty feet, if not more. The 

 gregariousness of the species is, of course, due chiefly to the pecu- 

 liar nature of the soil and situation which it affects ; but a number 

 of other causes also contribute in their respective degrees to the 

 same end. The more effective of the various causes may be thus 

 summarized : 



(a) The exclusive character of the soil and locality in which 

 alone the sissu can come up, only khair, Zizyphus and a few other 

 species being able to grow there in appreciable numbers. 



(/>) Its ability to stand extreme temperatures, especially frost, 

 which is obviously very severe in the low and, during the cold wea- 

 ther } moist situations where it grows. 



(c) Its profuse annual, rarely biennial seeding, and the dissemi- 

 nation in immense quantities of its seed by water in the flooded 

 lands in which it comes up. 



(d) The quick and easy germination of its se^ds, especially un- 

 der the circumstances in which their dissemination is effected; 



(e) The number and great vigour of the callum-buds in the 

 youngest seedlings, which enables these to continue establishing 



,/ 



themselves in spite of violent floods, erosion, deposit of new silt, &e* 

 (ft Its germination only in loose, freshly-deposited silt, in 



which the young seedlings, owing to their prior appearance, have 



not to contend with the roots of other species, especially of the 



grasses. 



(</) The absence of fires in such places during the time the 



plants are establishing themselves, there being hardly any grass 



present to burn. 



(/i.) The 'great length of the taproot of its seedling, and the 



great depth to which its roots can penetrate. 



