LIFE HISTORY OF PINUS 37 



Hillhouse ('oo) consider that these bladder-like appendages con- 

 sist of the outer part only of the extine, the extine having under- 

 gone cleavage at these two points. In studying the develop- 

 ment of these organs from their earliest beginnings, it appears to 

 me that the line of cleavage lies rather between the two coats of 

 the young spore. If it is not, then at the time that the micro- 

 spore leaves the parent-cell, the intine has not been developed, 

 or, if present, is so delicate that I have not been able to detect 

 it (fig. 48). 



Growth of the Microspore. — After its escape from the 

 mother-cell the microspore undergoes rapid growth, and the 

 outer surface of the spore becomes beautifully marked by the 

 formation of delicate, irregular ridges over the entire inner sur- 

 face of the extine, except along that portion which connects 

 the two wings on the concave or ventral side of the pollen-grain. 

 It is at this point that the pollen-tube later makes its exit, and 

 there is here no appreciable thickening of the spore- wall. These 

 ridges continue to grow and extend inward forming a very pretty 

 reticulated structure which is most distinctly apparent on the 

 walls of the wings ; along the convex or dorsal side of the 

 pollen-grain the reticulations are closer and the extine forms a 

 broad, deeply staining layer (figs. 50-54, plate V). This 

 irregular thickening of the extine is an admirable adaptation for 

 securing strength with slight increase in weight. 



When the young microspore attains to its mature-size, a par- 

 tial wall, extending along the back and for a longer or shorter 

 distance down the sides of the spore, becomes apparent within 

 the intine (fig. 54). It consists of a broad, homogeneous- 

 appearing band which gives precisely the same staining reac- 

 tions as the thick wall developed within the spore-mother-cell 

 after the formation of the young microspores. These immature 

 pollen-grains, after treatment with Flemming's triple combination 

 or with the gentian-violet and orange G alone, afford the most 

 brilliant effect that I have observed with these stains. The extine 

 presents a very intense, clear blue, the inner homogeneous wall 

 an equally vivid yellow, while the protoplasmic elements take 

 the colors characteristic for these dyes. The fact that this third 

 partial wall fails entirely to respond to some stains doubtless 



