142 A PINE. 



The needle-leaves are borne upon dwarf branches 

 that may be seen when the sheathing scale-leaves are re- 

 moved. It is to be noticed that the vascular bundles of 

 all the needle-leaves of a pair or group borne on the same 

 dwarf branch have their xylem portions facing a common 

 center, while their phloem portions are peripheral, as 

 would be true in an ordinary branch from the stem. The 

 imbedding of the vascular bundles in a group of colorless 

 tissue surrounded by a sheath is common among the 

 pines and their allies. Poorly developed resin-ducts 

 are occasionally found in the xylem of leaf-bundles, al- 

 though it has been denied by some writers. 



In comparing the reproduction of the pines with that 

 of the plants already studied we find advances of much 

 interest. In the Bryophytes and in the lower Pterido- 

 phytes the asexual spores upon germination develop 

 relatively prominent structures upon which sex-organs 

 eventually develop. These sex-organs produce gametes 

 and sexual spores from which grow sporophytes, upon 

 which the asexual spores are produced in their turn. 

 In the highest Pteridophytes the asexual spores are 

 unlike in size, produce unlike gametophytes, and after 

 germination retain the gametophytes within their walls. 

 Fertilization occurs even while the female gametophyte 

 is within the old cracked megaspore wall. In Selaginella 

 the embryo sporophyte not only begins its development 

 within the female gametophyte while this is enveloped 

 by the ruptured spore-wall, but often while the megaspore 

 still lies unshed, enclosed in the sporangium which pro 

 duced it. Furthermore, in these highest Pteridophytes 

 the sporophylls are collected into distinct cones. 



