202 
THIRTEENTH REPORT. 
which chickweed ( Stellar ia media ) was most abundant. The relative 
distribution of these plants is shown at “A” in the accompanying dia- 
gram. 
The spring of 1910 set in unusually early and by the end of the second 
third of April virtually all the trees had leaved out. The last third of 
April was ushered in by heavy wet snows and continued freezing 
weather. This resulted in the virtually complete defoliation of the bass- 
wood and consequent readmittance of light to the ground below. The 
basswood did not become refoliated until late in July. Meanwhile great 
changes were taking place on the ground beneath. Within a month 
the bluegrass had reinvaded not only the moss ring but also the bare 
ground at the base of the tree. It was quite well established before 
the leaves reappeared on the basswood. The amount of moss and chick- 
weed was noticeably diminished. Their zones were quite evidently re- 
treating. This condition of distribution is shown at “B” on the accom- 
panying diagram. 
When the basswood regained its leaves conditions were changed again. 
The bluegrass which had regained its lost ground was again vanquished. 
This was, however, not entirely completed during the growing season 
of 1910. The bluegrass was entirely driven out from the immediate 
vicinity of the tree trunk but not entirely out of the moss ring, as is 
shown at “ C ” in the diagram. 
This gives an idea of how rapidly invasion may take place beyond a 
tension line — such the boundary between sufficient and insufficient light 
— when the factor which causes it is decidedly changed. It leads to the 
conclusion that when one factor is varying the time of invasion is rela- 
tively much shorter than the time of retreat. 
University of Michigan, April 20, 1911. 
THE ORCHID FLORA OF THE VICINITY OF BATTLE CREEK. 
C. C. MC. DERM ID. 
(Names according to Gray’s New Manual.) 
1. Corallorhiza tritida. Rare. 
2. Corallorhiza maculata. Common. 
3. Cypripedium acaule. Plentiful. 
4. Cypripedium hirsutum. Rather common. 
5. Cypripedium parviflorum. Rather common. 
6. Cypripedium parviflorum var. pubescens. Rather common. 
7. Cypripedium candidum. Rare. 
S. Habenaria bracteata. Rather common. 
9. Habenaria lacera. Common. 
10. Habenaria leucophaea. Rather rare. 
11. Habenaria ciliaris. Common. 
12. Habenaria ciliaris x II. leucophaea. Rare. 
13. Habenaria Hookeri. Infrequent. 
