180 INTRODUCTION TO CYTOLOGY 



as well as such phenomena as conductance and polarity, are best explained 

 on the assumption that protoplasm itself possesses structural units 

 arranged in linear series, especially since certain other materials exhibiting 

 such properties are known to have such a structure (see Beutner, 1933). 



The Walls of Spores. — In bryophytes and vascular plants the walls of 

 spores are developed in two general ways: (1) by the successive formation 

 of layers within the original membrane by the protoplast, often with the 

 addition of material from the anther fluid, and (2) by the deposition of 

 material on the outside of the original membrane by a Plasmodium 

 formed by tapetal cells. 



In the first method a more or less temporary gelatinous layer is ordi- 

 narily developed around each of the young spores, either before or after 

 these have separated from one another (Fig. 96). Upon the inner surface 

 of this special layer the protoplast deposits the exine, or outer wall layer.^^ 

 In many cases this is at first a homogeneous layer that soon differentiates 

 into an outer lamella and an inner zone with net-like thickenings and 

 spines (the ''mesospore"). Finally, there is deposited an inner layer, or 

 intine. This mode of development is widely prevalent, the walls of most 

 spores showing two principal layers, or "coats": an intine, which later 

 becomes greatly extended to form the pollen tube in seed plants, and an 

 exine, which is characteristically thickened and sculptured. In angio- 

 sperm pollen the exine has one or more definitely differentiated germ 

 pores through which pollen tubes are to emerge. The characters of the 

 resistant exine are of much value in determining the relationships of 

 living and fossil plants. ^^ The exine, although it begins to differentiate 

 in contact with the protoplast, may continue to thicken and develop its 

 peculiar markings after it has been separated from the protoplast by 

 other layers.-^ In an extensive study of the composition of pollen-grain 

 walls Biourge (1892) showed that they contain cutin, callose, cellulose, 

 and pectic substances, singly or in various combinations. 



The highly specialized coats of the megaspore of Selaginella have been 

 repeatedly studied. In Selaginella rupestris^^ the coats begin to differ- 

 entiate in the midst of a thick gelatinous layer developed at the close of 

 the divisions delimiting the spores. The exospore first appears as a 

 double zone, the outer part of which becomes the perinium (Fig. 105) 

 The small protoplast now expands and pushes outward the undifferen- 

 tiated inner portion of the gelatinous layer, and, as it does so, a second 

 coat, the endospore, is formed at its surface. In *S. emiliana the exospore 

 and endospore develop simultaneously. Lyon finds two coats in place 



^^ E.g., Ipomoea purpurea (Beer, 1911). 



2^ E.g., Wodehouse (1928 et seq.) on living plants. For references on fossil pollen, 

 see Erdtnian (1927, 1930) and Sears (1932). 



"Beer (1905, 1911), Tischler (1908). 



2«Lyon (1905). The accounts of Fitting (1900, 1906) differ from this in certain 

 details. 



