132 MORPHOLOGY OF ANGIOSPERMS 



and Rhododendron i in other eases 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 poll in i am, 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 

 continued bv 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 lirst 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 chromatin 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 {i 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 



