GYMNOSPEEMS 325 



Life history. It will be seen from the above brief account of 

 the cycad that it has the same general stages in its life history 

 as Selaginella (Fig. 185). The new features are concerned prin- 

 cipally with the formation of a pollen tube, the changed rela- 

 tions of the megasporangium and megaspore, and the formation 

 of seeds. The retention of the megaspore in the sporangium, 

 which remains longer on the mother plant than in Selaginella, is 

 accompanied in the cycad by pollination and the formation of a 

 pollen tube to serve as an anchoring and absorbing structure 

 during the development of the motile male gametes. The per- 

 manent retention of the megaspore has also resulted in the 

 formation of a true seed composed of the megasporangium, the 

 garnet ophyte, the megaspore, and the embryo sporophyte. When 

 the seed germinates, the sporophyte resumes its growth and gives 

 rise to a new adult cycad plant, or sporophyte. 



CONIFERALES 



THE SPRUCE (PICEA) 

 SPOROPHYTE 



Habitat and habit. The spruce tree, which is the spore-bearing 

 plant, or sporophyte, has the same general form and mode of 

 growth as the pine tree described earlier in the text. Like the 

 pine the spruce is an erect tree type with an excurrent trunk 

 and pyramidal crown, which results from its mode of growth and 

 the spiral arrangement of its branches. 



The spruce differs from the pine in that its needlelike leaves 

 are borne directly and singly on the main shoot instead of in 

 pairs or clusters on the end of minute dwarf shoots. Like the 

 pines and their allies the spruces also inhabit mainly northern 

 or mountainous regions and are typically xerophytic in habit and 

 structure, although they adapt themselves readily to cultivation 

 and to mesophytic conditions. In stem structure the spruces are 

 intermediate between the pteridophytes and the woody-stemmed 

 flowering plants, as the following account of the structure of 

 the spruce will indicate. 



