132 MORPHOLOGY OF ANGIOSPERMS 



and Rhododendron; in other cases the whole product of a 

 primary sporogenous cell, ranging from eight to sixty-four 

 microspores, clings in a mass, as the massulae of certain orchids 

 (Ophrydeae) and the groups of pollen-grains found among the 

 Mimoseae; and in the most extreme cases, the whole product 

 of a sporangium forms a single mass, the pollinium, character- 

 istic of certain Orchids and of the Asclepiadaceae. It is of 

 interest to note that all of these conditions occur among Or- 

 chidaceae, from isolated microspores (Cypripedium) to the com- 

 pletely organized pollinium. Such variations and others have 

 been described in detail by Reichenbach, 1 Hofmeister, 3 Rosa- 

 noff, 4 Corry, 11 and others. 



The older botanists were not able to recognize the structures 

 developed within the mature pollen-grain, whose contents they 

 called " fovilla," regarding it as a fertilizing substance rich in 

 food material. In 1878 Strasburger 6 discovered that struc- 

 tures are developed in the microspores of Angiosperms com- 

 parable to those already known in Gymnosperms, and this was- 

 confirmed by Elfving. 7 



The germination of the microspore begins with the division 

 of its nucleus, and this always occurs before dehiscence, some- 

 times long before, the two daughter nuclei having been found 

 even in midwinter, as in Alnus and Corylus (Chamberlain 38 ) 

 (Fig. 8). When first formed, the daughter nuclei are usually 

 alike in size and form, but in most cases the tube nucleus soon 

 becomes much larger, the differentiation sometimes beginning^ 

 as in Cypripedium, before the mitosis is fully completed (Fig. 

 62). In any case, the nuclei soon become differentiated, the 

 tube-nucleus having a large nucleolus and a rather scanty chro- 

 matin network; while the generative nucleus is smaller, has a 

 smaller nucleolus or none at all, and its chrbmatin is denser 

 and less irregular. The nuclei also differ in their reaction to 

 stains, a combination like cyanin and erythrosin staining the 

 tube-nucleus red and the generative nucleus blue. 



At first Strasburger 6 thought that the tube-nucleus was 

 concerned not merely in developing the pollen-tube, but also in 

 fertilizing the egg, and hence named it the " generative nu- 

 cleus." The other nucleus, although seen to enter the tube and 

 even divide, was thought to take no part in the processes con- 

 nected with fertilization, and was called the " vegetative " or 



