THE MALE FLOWER OF PINUS. 369 



vascular bundle, the insertion of the pollen-sacs on the staminal 

 leaves. Upon fragmentary longitudinal sections, thinner parts 

 can be readily found, in which the structure of the individual 

 staminal leaf (B) may be still better followed. 



We next prepare tangential longitudinal sections through the 

 flower, in order to obtain cross-sections of single staminal leaves, 

 and pick out such an one for closer study (C). We see that the 

 two pollen-sacs adjoin in the middle line, and, when ripe, are 

 usually separated only by a flat wall of collapsed cells, in which 

 one or more layers of flat starch-containing cells may be medianly 

 interposed. Upon their free outer surface the pollen-sacs are 

 covered by the epidermis, to which, towards the interior, usually 

 only collapsed cells adjoin ; towards the dorsal surface of the 

 leaf, likewise, the pollen cavities are enclosed in the same way. 

 In the median line of the anther, above and below the partition 

 wall separating the two pollen-sacs, runs a strip of mesophyll. 

 The upper of these is thicker, and is traversed by the very delicate 

 vascular bundle. At the two side edges of the anther, the epider- 

 mis projects into a more or less feebly-developed wing ; where 

 this is thickest, a little mesophyll can be found between the two 

 layers of epidermis (C). On the under side of the pollen-sacs 

 the epidermal cells diminish in size from both sides ; at the 

 places of weakest development the pollen-sacs open. These 

 pollen-sacs closely resemble the sporangia of Lycopodiaceee ; 

 researches in comparative development have, in fact, led to the 

 conception that the pollen-sacs of Phanerogams, and the micro- 

 sporangia of Cryptogams are homologous structures, and that the 

 pollen-grains, therefore, are microspores. 



The Pollen-Grain. If we look now to the pollen-grains 

 developed in the pollen-sacs, where possible in the fresh state, we 

 shall note that each of these consists of a central body, upon 

 which are placed laterally two bladdery vesicles (D). If the 

 flower is ripe, the two vesicles appear dark, because filled with 

 air. They show a delicate tracery upon their surface. The 

 middle, true pollen-cell, contains finely granular protoplasm, and 

 a large nucleus. Shortly before dehiscence, i.e., before the 

 opening of the pollen-sacs, a division takes place in the pollen- 

 grain, and in the ripe pollen- grain we see, in the middle of the 

 convex side, turned away from the two vesicles, a lenticular cell 

 adpressed to its wall. This cell is best seen when the pollen- 

 grain, as in our figure, lies on its side. In the pollen-grains of 



24 



