134 BOTANY. 



a day or two open, often with a sharp, crackling sound, and 

 scatter the ripe seeds. 



A section of a ripe seed (F) shows the embryo (em.) sur- 

 rounded by a dense, white, starch-bearing tissue derived from 

 the prothallium cells, and called the " endosperm." This fills 

 up the whole seed which is surrounded by the hardened shell 

 derived from the integument and wall of the ovule. The 

 embryo is elongated with a circle of small leaves at the end 

 away from the opening of the ovule toward which is directed 

 the root of the embryo. 



The seed may remain unchanged for months, or even years, 

 without losing its vitality, but if the proper conditions are 

 provided, the embryo will develop into a new plant. To follow 

 the further growth of the embryo, the ripe seeds should be 

 planted in good soil and kept moderately warm and moist. 

 At the end of a week or two some of the seeds will probably 

 have sprouted. The seed absorbs water, and the protoplasm 

 of the embryo renews its activity, beginning to feed upon the 

 nourishing substances in the cells of the endosperm. The 

 embryo rapidly increases in length, and the root pushes out 

 of the seed growing rapidly downward and fastening itself in 

 the soil (G, r). Cutting the seed lengthwise we find that the 

 leaves have increased much in length and become green (one 

 of the few cases where chlorophyll is formed in the absence 

 of light). As these leaves (called "cotyledons" or seed leaves) 

 increase in length, they gradually withdraw from the seed 

 whose contents they have exhausted, and the young plant 

 enters upon an independent existence. 



The young plant has a circle of leaves, about six in number, 

 surrounding a bud which is the growing point of the stem, and 

 in many conifers persists as long as the stem grows (Fig. 75, 

 K, b). A cross-section of the young stem shows about six 

 separate fibro-vascular bundles arranged in a circle (8, fb.). 

 The root shows a central fibro-vascular cylinder surrounded by 



