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 (Ophioglossace^e) 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 vei'y 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 corms 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 corms* 

 protected as they usuallj^ 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 off in the Spring by 

 the expanding leaves of the ti'ees. 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. 



