164 TYPE STUDIES 



development is that of the pollen tube, which carries the sperm nuclei to the 

 embryo sac. Microtome sections of the ripe anthers of the elder (Sec. 212) 

 will give excellent preparations of this interesting condition. 



The pollen grains of many plants, as the lily, contain but tw^o nuclei at 

 the time of pollination. These are the tube nucleus, above mentioned, and 

 the generative nucleus, which later gives rise to the two sperm nuclei. 



143. Capsella, the shepherd's purse, studied for the development of the flower, 

 ovule, pollen tube, and embryo (App. 23). The shepherd's purse is in many 

 respects an excellent type for a general study of a dicotyledonous seed 

 plant, although the flower is rather small. It is a particularly good subject 

 for the study of the topics indicated in the above heading. 



A. The development of the flower. Tease apart in a drop of water, under 

 a dissecting microscope, the extreme tip of a flower cluster (which is a 

 raceme), to obtain the youngest flowers (microscopic) just below the 

 growing point. Older stages may be cleared by adding potash solution 

 if necessary (Sec. 160). Search for the following stages and draw : 



1. The young flowers appearing as small protuberances just back of the 

 growing point. 



2. A circle of sepals developing at the tip of the young flower. 



3. The growth of the sepals over the tip of the flower and the origin of 

 the stamens in a circle within. 



4. The development of two carpels, more or less united below, at the 

 tip of the flower. 



5. The late appearance of the petals between the sepals and stamens, 

 after all the other parts of the flower are present. 



6. The final folding one over the other of the sepals in the young bud, 

 the growth of the petals, the differentiation of the stamens into 

 anthers and stalks (filaments), the union of the carpels above to form 

 the pistil, with the ovule case or ovary below, in which the ovules ap- 

 pear as outgrowths along the surface. 



7. Microtome sections cut lengthwise of the tip of the raceme (Sec. 212) 

 give excellent stages in the development of the flowers, and especially 

 the closing together of the carpels to form the pistil and the origin 

 of the ovules along their inner surface within the ovule case. 



B. The development of the ovule. Pick to pieces some of the youngest 

 flowers that can be easily seen with the unaided eye. Open the ovule 

 cases, or ovaries, in a drop of water, with the point of a needle, thus 

 exposing the ovules. A variety of stages will be presented. Search for 

 the following and draw : 



1. The young ovule, consisting of the nucellus at the end of a short 

 stalk, with the two integuments just beginning to appear like collars 

 at its base, 



