THE AMERICAN BOTANIST. 79 



when growing on a flat surface the ivy shape enables the leaves 

 to fit into one another and to cover the whole surface. On the 

 other hand the flowering branches grow up into the air. The 

 leaves are arranged round the stem and under these conditions 

 an oval form is more suitable. According to the same writer 

 the interior of the leaf differs according to the amount of light 

 it receives. In the sun two layers of palisade cells are develop- 

 ed under the upper epidermis while in the shade the whole in- 

 terior of the leaf consists of rounded cells. The same thing 

 is said to be true of the leaves of the common dandelion. 

 Ed.] 



The Pollination of Yucca. — The pollination of the 

 various species of yucca by the pronuba moth, although well 

 known is a never failing source of wonder and interest to the 

 flower lover. Here we have a flower whose six stamens are 

 far too short to be of use in pollinating the individual blossom 

 in which they live, and a pistil whose receptive surface is so 

 located that neither the wind nor the ordinary visiting insect 

 is effective in pollination. At this juncture the special guar- 

 dian of the flower, the pronuba moth comes in. Gathering 

 the pollen from the anthers by mouth parts specially formed 

 for the purpose she deliberately flies to another flower, climbs 

 up to the stigmatic chamber and packs the pollen into it with 

 a sort of hammering motion that is plainly visible to the ob- 

 server. Without these ministrations of the moth, the yucca is 

 incapable of setting seed and its distribution or spread in the 

 wild state is absolutely dependent upon this, otherwise insig- 

 nificant insect. All this is wonderful enough but not half so 

 interesting as the questions it raises. Back of the way in which 

 pollination is effected lie the speculations as to how this asso- 

 ciation of insect and flower have been brought about. When 

 did the species learn that it could safely trust its continued ex- 

 istence to the ministrations of an animal and how did it happen 

 that the body of this insect was modified at just the right time 

 and in just the right w^ay to be of service to the plant? To be 



