334 



GENERAL MORPHOLOGY OF PLANTS 



the leaves and leaf stalks, and provides for transport of water 

 and food solutions. The development of the leaf also differs, in 

 that the basal portion develops first and the apical portions are 

 successively developed. The leaf is circinate in its development. 

 The leaf is coiled, and as it develops it gradually uncoils. This 

 is very striking in some of the large ferns grown in greenhouses, 

 but is also easily observed in our ordinary ferns. 



LIFE HISTORY OF FERNS. 



504. The prothallium and sexual organs. The spores 

 from the spore cases germinate and produce the prothallium, 



Fig. 312. 



Spore of Pteris serrulata, 

 showing the three-rayed 

 elevation along the side of 

 which the spore wall cracks 

 during germination. 



which is the first gene- 

 ration or gamete plant. 

 At first a short thread of 

 two to three cells, con- 

 taining chlorophyll, is 

 developed from the 

 spore. This is called the 

 protonemal thread. A 

 rhizoid is developed from the first cell of this thread. The ter- 

 minal cell divides in two directions, forming a flattened green 

 body, the young prothallium. Later it becomes more or less heart- 

 shaped in outline, one cell layer in thickness on the sides, but 

 several cells thick over the middle part, forming a thin cushion. 

 Rhizoids are developed from the underside near the smaller (pos- 



Fig. 313- 



Germinating spores of 

 Pteris, aquilina still in the 

 sporangium. 



