ADJUSTMENT TO TEMPERATURE 115 



stem, which forms roots and leaves at its outer end, either before 

 or after the latter touches the ground. Offsets are short runners, 

 but there is no real difference between the two. Runners are found 

 in the strawberry, certain species of erigeron, saxifrage, etc. 

 Rhizomes or rootstocks are the underground stems or branches 

 so characteristic of perennial herbs, e.g., most grasses, mints, 

 iris, Solomon's seal, etc. They persist from year to year, forming 

 new buds and carrying the successive generations further and 

 further from the original home. A tuber is a greatly thickened 

 rootstock, or a thickened portion of one, such as is found in the 

 i:)otato. The corm is really a short tuber, often broader than 

 long, and usually upright in position. Roots are produced from 

 the lower surface and buds from the upper, though they may 

 spring from the sides also. Corms are found in the crocus, jack- 

 in-the-pulpit, etc. Bulbs resemble corms in shape, but they are 

 not solid. They are greatl}^ shortened subterranean stems, made 

 up largely of thickened scale-like leaves. Bulbs are the charac- 

 teristic propagules of the lilies and their relatives. Bulbs and 

 corms sometimes form underground offsets which produce new 

 bulbs at the end. Bulbils or bulblets are small aerial bulbs, 

 produced in the axils of leaves, as in the lily, or in flower clusters 

 as in the onion. 



Experiment 37. Propagules. Note the development of the porcnnial 

 herbs and the shrubs in the spring. Determine the method of pro]:)aga- 

 tion in as many as possible. Prepare a list for grassland and forest 

 of the plants thus studied. Arrange the species according to the type 

 of propagule, and note the distribution and importance of the various 

 types. 



139. Sexual reproduction. In its simplest form sexual re- 

 ])ro(luction is merely the fusion of two protoplasts or gametes. 

 There is no differentiation of the gametophore, and fertilization 

 has no effect apart from the two cells concerned. In the beginning, 

 sexuality seems to be little more than a device by which a double 

 quantity of protoplasm is secured for the resting spore. In the 

 carpophytes, and especially in the l)ryophytes, the gametophore 

 is considerably differentiated. I'ertilization produces a sporo- 

 phore of increasing complexity, in which spore production, though 

 still the principal function, is not the only one. Stej) by step 

 the sporophore assumes the functions of the gametophore, until 



