GERMINATION 



931 



early germination of seeds within the fruit 

 (as in the lemon). The cause of vivipary is 

 unknown, though if seed formation results 

 from increasing xerophytism or from decreas- 

 ing nutrition, vivipary may be due to the 

 continuance of conditions favorable to vege- 

 tative development or to the inception of 

 such conditions at fruit maturity. This idea 

 seems to be favored by the fact that various 

 grasses exhibit vivipary in wet autumns, and 

 that peas and beans, when vegetative con- 

 ditions are favorable, often exhibit uninter- 

 rupted embryo development. Approaching 

 such vivipary is the germination of seeds while 

 still within the fallen fruits of Typha and 

 A ndropogon. 



Seed maturity and germination. — A 

 number of seeds are capable of germi- 

 nation as soon as they are shed; among 

 such are those of the willows, the sen- 

 sitive plant, and many cycads, crucifers, 

 and grasses.' It is a matter of com- 

 mon belief, however, that most seeds 

 require a resting period of some weeks 

 or months before they are capable of 

 germination, and that in temperate and 

 in cold climates germination ensues 

 only after a period of rest in the 

 ground, coupled with exposure to low 

 temperatures. In many seeds under 

 ordinary conditions the germinative 

 capacity may improve with age, a cer- 

 tain percentage being capable of germi- 

 nating after the first winter, a larger 

 percentage after the second winter, and 

 in a few instances a still larger percent- 

 age after the third winter; it is said 

 that the seeds of certain conifers are 



^ It will be recalled that willow seeds soon lose 

 their vitality, especially if desiccated; seeds of 

 the sensitive plant, however, have been known 

 to retain their vitality for sixty years. 



Figs. 1225-1227. — Vivipary in the 

 mangrove (Rhizophora Mangle) : 

 1225, a mature fruit attached to the 

 tree, the basal portion of the embryo 

 (r) just emerging; 1226, a later stage 

 in which the young plant has become 

 so heavy that it falls from the parent 

 tree ; note the plumule (p) and the 

 greatly enlarged basal portion of the 

 embryo (r) ; 1227, a stage still later, 

 in which the young plant has rooted 

 freely in the mud (;•'), the plumule 

 meanwhile having grown vigorously 



