Dec., 1900] 
Tyler — Geophilous Plants. 
23 
way. Trillium nivale Riddell is in a transitional stage between the 
progressive and retrogressive classes, since the large rhizomes are 
upright and the young lateral branches are progressive until they 
have traveled some distance away from the parent rhizome, when 
they too, become upright. The lower Ferns (Ophioglossaceae) belong 
to this class. The disadvantage of this habit is that the rhizome will 
soon grow out of the ground and be in a very exposed condition. 
To counteract this tendency the roots of these plants are usually 
strongly contractile and pull the rhizome down into the ground as 
fast as it grows out. Skunk Cabbage (Spathyema foetida (L.) Raf.) 
has an upright rhizome and root contraction is very marked. The 
very apparent disadvantages of the retrogressive or upright rhizome 
habit have made this class very few in number compared with the 
progressive rhizome class. In Ohio there are about 475 species of 
rhizome plants and less than twenty-five of these belong to the retro- 
gressive class. This class is closely related to the corm plants, 
indeed, all that is needed to make the typical corm out of a retro- 
gressive rhizome plant, such as Trillium nivale, is to shorten and 
make more definite the annual growth of the rhizome. The bulb is 
usually a very short, upright rhizome with many thickened scales. 
The bulb of Lilium martagon is of this kind but that of Lilium cana- 
dense is more closely related to the progressive rhizomes. The 
parent bulb sends out one or more thick rhizomes which grow out- 
ward if the bulb is at the normal depth, downward if the bulb is too 
near the surface of the ground and the new bulbs are formed by the 
shortening of the outer end and the growth and thickening of the 
scales of the rhizome. 
Both bulbs and conns may be regarded as rhizomes modified to 
suit peculiar conditions, such as a long, dry, heated period alternat- 
ing with a short, rainy period. A plant to survive under such 
conditions must be able to start up very quickly as soon as the rains 
come, and flower and mature its seeds before the drouth again over- 
takes it. A large amount of food material must be stored up by the 
plant in order to do this, and the food material must be kept from 
drying or burning up during the heated period. Bulbs and cormsi 
protected as they usually are by dry and coriaceous coverings, ans- 
wer these requirements and are usually abundant in localities where 
these conditions obtain. Bulb and corm plants are also well fitted to 
live in dense woods where the light is soon shut otf in the Spring by 
the expanding leaves of the trees. They are able to spring up very 
early, flower and ripen seeds before the light is shut off. The food 
supply which enables them to do this is often protected by acrid or 
poisonous principals developed in the bulb or corm. Pepper-root 
(Dentaria laciniata Muhl.) and Jack-in-the-Pulpit (Arisaema tri- 
phyllum [ L. ] Torr.) are examples. 
