290 INTRODUCTION TO BOTANY 



are made up chiefly of stamens, which bear large pollen sacs 

 (fig. 225) from which pollen is shed in great quantities. It 

 may be carried long distances, and its transportation may be 

 facilitated by peculiar wings, or outgrowths, upon the walls 

 of the pollen grains. Sometimes the pollen is shed in such 

 great abundance that some people have thought it was a 

 shower of sulphur from some distant volcano. 1 



275. Pollination. As was learned in the chapter on polli- 

 nation and fertilization, the pollen (or microspores) must be 

 placed on the part of the flower which bears the ovules before 

 the pollen grows. It was also learned that the proper placing 

 of the pollen is known as pollination, and considerable study 

 was made of the different ways in which pollination occurs. 

 In the pines the young seed cones stand upright and open 

 (fig. 220) at the time the pollen is being shed. If pollen grains 

 chance to come into the seed cones, they slide down upon the 

 leaf -like parts to the base, where the sporangia are borne. By 

 means of a sticky secretion they are then caused to adhere to 

 the tip of the ovule, and pollination is completed. In order 

 that pollination in pines may be complete, it is evident that 

 the air must be full of pollen when the ovules are ready to 

 receive it ; and this is the case. The pollen is not only very 

 abundant but, because of its lightness, is easily transported. 



276. Fertilization. In the ferns the egg is borne in the 

 archegonium, the neck of which opens to the exterior, so that 

 the sperm may swim directly into the passageway leading 

 to the egg. In the gymnosperms, on the contrary, the egg is 

 imbedded within the tissue of the ovule. After the pollen 

 grain falls upon the ovule, there grows from its wall a tube 



1 Unfortunately for the learner, each of the structures that compose the 

 staminate cone has had two or more names applied to it, all of which are in 

 use in botanical writings. The pollen grains are also known as microspores 

 (small spores) ; the pollen sacs are called microsporangia (small spore sacs) ; 

 the stamens are called microsporophylls (small spore leaves) ; the whole stam- 

 inate flower is called the strobilus, a name which was used for the cone of 

 the club moss and in a way distinguishes it from the type of flower that is 

 found in the angiospenns. 



