196 INTRODUCTION TO CYTOLOGY 



than three, but points out that a portion of the spore membrane which 

 may remain in an undifferentiated condition until a late stage may easily 

 be mistaken for a third coat. The two "spaces" in the immature spore 

 wall she holds to be undifferentiated regions in the spore membrane, and 

 not cavities filled with a foreign fluid; and further urges that the proto- 

 plast is at all times in contact with the gelatinous spore membrane in 

 which the coats are differentiating, opposing the view that the latter 

 have the power of independent growth in thickness. 



Evidence favoring the view that the spore coats can grow while not 

 in contact with the protoplast has been brought forward by Beer (1905, 

 1911) and Tischler (1908). Beer asserts that although both the primary 

 wall and the secondary thickening layer of the pollen grain (in certain 

 members of the Onagraceae) originate in intimate connection with the 

 plasma membrane, most of their subsequent growth occurs by intussus- 

 ception while they are completely separated from the protoplast, which 

 secretes the material used. The development of the pollen wall in 

 Ipomoea purpurea has been described in great detail by Beer. Around 

 each young spore immediately after its formation there appears a tem- 

 porary gelatinous "special wall," upon the inner surface of which the 

 protoplast deposits the exine, or outer spore coat. This is at first homo- 

 geneous, but soon differentiates into a thin outer lamella and an inner 

 zone made up of a network of thickenings with the rudiments of spines 

 at its nodes. Both the spines and the small rodlets, which develop in a 

 clear space appearing between the outer lamella and the network of 

 thickenings (mesospore) , undergo most of their development after they 

 are separated from the protoplast. Tischler (1908) reports that the 

 exine of the pollen of sterile Mirdbilis hybrids may continue to increase 

 in thickness after the protoplast begins to degenerate. 



As an example of the formation of spore coats through the activity 

 of a tapetal plasmodium may be taken the case of Equisetum, described 

 by Beer (1909) and Hannig (1911). The spores of this form have three 

 coats: an endospore, an exospore, and a perispore consisting of several 

 layers including the one which splits to form the "elaters." The young 

 spore cell at first has a simple membrane, the rudiment of the exospore. 

 The walls of the tapetal cells dissolve, allowing the cell contents to flow 

 freely among the spores as a tapetal plasmodium. Upon the spore 

 membrane the plasmodium deposits successively (1) an inner gelatinous 

 layer, (2) the "middle coat," (3) an outer gelatinous layer, and (4) the 

 elater layer. The exospore develops from the original membrane after the 

 middle coat is formed, and the endospore, or innermost coat, is developed 

 last of all. 



From this brief review, to which other examples might be added, it is 

 evident that spore coats may develop in a variety of ways, but too little 

 is known to warrant any statement as to which method may be the most 



