434 



PART III. — THE CLASSIFICATION OF PLANTS. 



The dehiscence is mainly effected by a layer of tracheidal cells, 

 differentiated as part of the wall, which are highly hygroscopic. 



The microspores, or pollen-grains, are developed from the sporo- 

 genous mother-cells of the sporangium. As a rule each mother-cell 

 divides so as to give rise to four microspores, all of which develope. 

 As a rule, also, the microspores eventually become quite free from 

 each other, bat to this there are exceptions: thus, in the Mimoseas, 

 while the pollen-grains are isolated in some species, in other 

 species they cohere in groups of 4, 8, 12, 16, or 32, derived from 

 one, two, three, or more mother-cells; again, in the Orchidacese, 

 whilst Cypripedium has isolated pollen-grains, in most genera the 

 pollen-grains are in groups of four (tetrads), and cohere into a 

 mass (or 2-8 masses), the pollinium, of varying consistence (see 

 Orchidaceae) : poUinia also occur in the Asclepiadaceae. 



The microspore has, as a rule, the ordinary structure of a spore 



(see p. 69) ; it is a nucleated 

 cell, with a certain amount 

 of granular nutritive mate- 

 rial in its cytoplasm, and 

 has two coats, an intine and 

 an exine, the structure of 

 the latter being elaborate 

 in many cases. The spore 

 has not, however, always 

 two coats. In some plants 

 there is no exine, and only 

 a single thin coat, as in the 

 cells of the poUinia of Or- 

 chids and Asclepiads, and 

 in certain plants whose 

 flowers develope under water, such as certain Naiadaceas (Zos- 

 tera, Posidonia, Cymodocea, Naias), in Halophila (Hydrochari- 

 daceoe), and in some species of Callitriche. In others, again, 

 there is but one coat, but it is thick and is cuticularised externally 

 (e.g. Senecio), or the two coats are only distinguishable at those 

 points at which the pollen-tubes will be eventually protruded (e.g. 

 OnagracesB, Cobasa). 



The exine is frequently highly differentiated with special refer- 

 ence to the protrusion of the pollen-tubes : it may be porous (e.g. 

 Malvaceae, Fig. 283) ; or there may be thin areas at certain points ; 

 or (Onagraceae) much-thickened areas where the pollen-tubes are 



Fig. 282.—^ Pollen-grain of Cucurlita Pepo, 

 showing the lid-like areas through which the 

 pollen-tubes will protrude (x240). B Section of 

 one of these areas ( x 540: after Strasburger). 



