148 



GENERAL BIOLOGY 



Fig. 93. Cross section of com 

 stalk, c, cortical layer; v, 

 vessels scattered through 

 the parenchyma. 



under ground stems of the dominant species are not in- 

 jured by fires that would hinder forest growth. 



From this very brief sketch of 

 the seed-plant sporophyte we may 

 learn that the parts of the plant 

 body are much the same as were 

 found in pteridophytes — -only 

 modified in form and arrange- 

 ment. Rhizoids and chlorophyl 

 bearing cells are the foraging 

 agents still, and vascular bun- 

 dles, the means of communication 

 between them. 



Development. — When we turn 

 to the developmental side the 

 differences are much greater. 

 Flowers appear in the sperma- 

 tophytes, and — what is vastly more important — seeds, 

 also. We have already seen (Chapter I) something of the 

 variety of floral structure. We know that the purpose of 

 the flower is to produce seed. Let us now study the manner 

 in which seeds are developed. 



Figure 94 shows at a the flower of the chickweed with its 

 three stigma-tips alternating with three small stamens, and 

 with five small white bifurcated petals wholly encompassed 

 by a like number of big green sepals. At h in the figure, 

 surrounded by the persistent sepals and surmounted by the 

 stigmas, is shown the maturing fruit, within which the seeds 

 are contained. 



The differences between flowers of this type and the 

 pteridophyte in reproductive methods are so. great, that we 

 will find it easier to study first the conditions found in a more 

 primitive seed plant, and afterward, those found in a highly 

 developed flower. So let us examine first the pine, and 

 after that the violet. 



