934 ECOLOGY 



suitable host. The exact factor here concerned is not known, though it may be 

 chemical in nature ; in certain orchids concentrated solutions may replace the usual 

 symbiotic fungus. Spores in general require germinative conditions similar to those 

 of seeds. Where the spores are green, as in mosses and ferns, light generally is re- 

 quired for germination, though in most cases germination may be induced in the 

 darkness by proper chemical stimulation. Many fungus spores, especially those of 

 parasites, germinate readily in water, but the spores of many saprophytes require 

 for germination the presence of a nutrient medium. 



The germinative processes of seedlings. — Initiatory activities. — 

 In those seeds in which the outer layer becomes transformed into muci- 

 lage upon the absorption of water (as in flax and mustard), this is the 

 first germinative phase to be observed. Very soon the seed swells notice- 

 ably, owing to the large amount of water absorbed. The outermost 

 cells become active, as soon as their contents become suflB.ciently dilute; 

 diastase or other enzyms are secreted, and the digestion of the accumu- 

 lated food begins. When the water and the transformed foods reach 

 the embryo and incite it to its second and final period of activity, ger- 

 mination proper may be said to have begun. 



The digestion and the absorption of foods. — The digestive processes 

 are observed readily in the grains of cereals, as in wheat. The aleurone 

 layer (fig. 12 ii), which is rich in protein, first shows signs of life, the cells 

 becoming large and vacuolated and the protoplasm manifesting activity. 

 Soon the secretion of diastase begins, and the starch next to the aleurone 

 layer is the first to be digested. In wheat, maize, and other grasses there 

 is a specialized structure, the scutellum (structually the cotyledon), 

 which greatly facilitates the germinative processes, since it serves as a 

 path of transfer for the digested foods from the endosperm to the devel- 

 oping embryo; often on the side next to the endosperm there are hair- 

 like absorptive cells. In many monocotyls the tip of the cotyledon 

 remains in the seed in contact with the food and may be regarded as an 

 absorptive organ (fig. 1229). In the date the tip of the cotyledon 

 enlarges into a disk, presenting a large absorptive surface to the endo- 

 sperm (fig. 1230). In those seeds in which the accumulated foods are 

 in the cotyledons, specialized absorptive structures are less likely to be 

 present. 



The amount of food in seeds may vary from almost none (as in many parasites 

 and mycophytes) to such large quantities as are found in the coconut and the 

 avocado (Persea gratissima). In cases like the latter, much or little of the food may 

 be utilized, depending upon the conditions to which the seedling is exposed 

 upon emergence from the testa. If the radicle has ready access to moisture and 



