CLUSTERED ARRANGEMENT. 



799 



in the light and in open air than in those which develop under the water or the 

 soil, or, perhaps we should rather say, that above the ground the greater fluctuations 

 in light, moisture, and temperature bring about corresponding modifications in the 

 vital processes. Moreover, the substratum presents every imaginable gradation 

 from shifting quicksand to heavy clay, from steep rock-faces in one place to the 

 bark of old tree-trunks in another, all these having by no means the same eflect on 

 the formation of offshoots. One of the most noteworthy processes occurring above 

 the ground leading to the formation of clustered offshoots is that exhibited by 

 Moss-protonemas. By protonema is meant a web of threads which spreads some- 

 times as a loose, open network, sometimes as a thick felt, over rock, clay, sand, earth, 

 humus, bark, and decayed wood, the individual cells becoming the starting-points 



445. — A section through soil permeated by the protonemal threads of the Moss Pottia intermedia. (Magnified.) 



of new Moss stems. This protonema may be compared to a web of Strawberry 

 runners which has spread over the ground in a wood-clearing. Just as small plants 

 spring up from the thread-like runners in this case, so Moss-plants are produced 

 from the protonemal threads, and by the dying away of the latter become isolated. 

 In many Mosses the end comes with the formation of this clustered arrangement, 

 as, for example, in the tiny Mosses classed together as Pottiaceae, of which one 

 species, Pottia intermedia, is shown in fig. 445. This plant has the following 

 remarkable history. During the period when most other plants are engaged in 

 active nutrition and reproduction it remains with its rhizoids and part of the 

 protonemal threads imbedded in the ground. Numerous scattered spores also 

 remain resting in the ground until at length the time for aerial development 

 arrives. Strangely enough, however, this is not until late in the autumn, when the 

 leafy trees have discarded their foliage and autumnal mists drift through their bare 

 branches. Then on the surface of the bare, cold, damp earth appear green threads 

 which at first look like algal filaments, and on these small buds are formed (see 

 fig. 445). In the course of a few weeks Moss-plants grow up from these buds 



