70 ECESIC CAUSES. 



for long periods are inimical to germination. In many cases, of course, the 

 surface layer favors germination, and deeper layers, persistence. 



Successful germination usually occurs only at proper depths, with the excep- 

 tion of bare areas with wet or moist surfaces. A few species have the peculiar 

 property of being able to plant themselves when they germinate on the sur- 

 face, but the rule is that seed? must be covered with soil to permit ecesis. 

 This is particularly true of seedsj)n a forest-floor covered with a thick layer 

 of leaves or needles, which prevent the root from striking into the soil. There 

 is doubtless an optimum depth for each species, which varies more or less 

 with the habitat. Too great a depth prevents the seedling from appearing 

 altogether, or causes it to appear in such abnormal condition that it quickly 

 succumbs. In the former case it may lead to dormancy, and germination 

 after the area has been cleared or burned. The effect of depth and its relation 

 to size of seed has been shown by Hofmarm (1916) in the case of conifers. In 

 Pinus ponderosa, with the largest seeds, 96 per cent germinated and 86 per 

 cent appeared above ground at a depth of 1 inch, whUe only 36 per cent ger- 

 minated and none appeared at 4 inches deep. In the case of Pseudotsuga, 93 

 per cent germinated and appeared at 0.5 inch, but only 17 per cent germinated 

 at 4 inches and none appeared. For Tsuga heterophylla, at 0.25 inch the per- 

 centage was 96 and at 1.25 inches 42 per cent and 0, and for Thuja plicata, 

 with the smallest seeds, 78 per cent at 0.12 inch and 26 per cent and at 1 inch. 

 The same investigator found that seeds of Pinus monticola, Pseudotsuga, and 

 Tsuga heterophylla remained dormant in the soil for 6 years, those of Taxus brev- 

 ifolia for 8, Abies amabilis for 5, A. nobilis for 3, and Thuja plicata for 2 years. 

 While this is a relatively short time in comparison with the period in some 

 ruderal species, it is of much more significance in succession (plate 16, b, c.) 



Fate of seedling. — ^The crucial point in ecesis is reached when the seedling is 

 completely freed from the seed-coat and is thrown upon its own resources for 

 food and protection. Even before this time invading seedlings are often 

 destroyed in great numbers by birds and rodents, which pull them up for the 

 food supply still left ta the seed-coats. The tender seedlings are often eaten by 

 the smaller chipmimks, and sometimes coniferous seedlings seem to be pulled 

 up or bitten off in mere wantonness. In regions where grazing occurs, the 

 destructive action of the animals is very great, especially in the case of sheep. 

 Some toll is taken by damping-off fungi, such as Pythium and Fusarium, in 

 moist, shady soils, but these are perhaps never decisive, except in artificial 

 conditions. In the case of herbs, the greatest danger arises from excessive 

 competition, especially in the dense aggregation typical of annuals. The 

 direct effept is probably due to lack of water, though solutes and light may 

 often play a part. With the seedlings of woody plants the cause of the 

 greatest destruction is drouth in midsmnmer or later. This is the primary 

 factor in limiting the ecesis of many conifers, though the "heaving" action 

 of frost is often great or even predominant. The root-system is often 

 inadequate to supply the water necessary to offset the high transpiration 

 caused by conditions at the surface of the soil. Moreover, it is likewise too 

 short to escape the progressive drying-out of the soil itself. In open places 

 in the Rocky Mountains, such as parks, clearings, etc., the late simuner mor- 

 tality is excessive, often including all seedlings of the year. On the forest- 

 floor itseff it is considerable or even decisive in places where a thick layer of 



