1J2 PART II. VEGETABLE HISTOLOGY. 



4. The rejuvenescence of cells may be observed by patiently studying the 

 filaments of such fresh water algae as Stigeoclonium, Oedogonium, and some 

 others. This process, as it occurs in Oedogonium, is illustrated in Fig. 396, a 

 and b. 



5. For the study of conjugation, select masses of Spirogyra late in the 

 season, when they have acquired a brownish color and a crinkled appearance, 

 and examine them under a moderate magnifying power. Every stage in the 

 process may often be seen in a single field of the microscope. Make drawings 

 of the successive stages. 



The same process may also be conveniently studied in Mesocarpus, another 

 filamentous alga, and in the Diatoms and Desmids. All of these plants are 

 abundant in our fresh waters. 



6. The process of fertilization may best be studied in the Vaucherias, or 

 Green Felts, filamentous algse found growing in intricate, felt-like masses in the 

 vicinity of springs, on damp rocks, or on the surface of wet, spongy soil. 



The process may be studied without great difficulty in the flowering plants, 

 as follows : 



Take the pistil of Datura Stramonium (or of some other flower whose ovary, 

 style and stigma are rather large), and, immediately after the withering of the 

 corolla, make thin, longitudinal sections through the stigma, style and ovary. 

 Pollen grains will ordinarily be seen attached to the stigma, and pollen tubes 

 may be traced into the style, particularly if the section be heate'd for a few 

 moments in glycerine, so as to render the cells of the style as transparent as 

 possible. 



The pollen tube may be distinguished by its more granular contents from 

 the cells among which it has penetrated. 



If the sections are fortunately made, the pollen-tube may even be traced 

 into the micropyle of the ovule. But if this cannot be done, some of the young 

 ovules, if removed and carefully examined, will be found to contain the ends of 

 pollen-tubes which have penetrated the micropyle and are in contact with the 

 nucleus of the ovule. If the ovary of almost any species of the Orchidace£e be 

 cut open longitudinally a short time after the corolla has withered, numerous 

 pollen-tubes will be seen, appearing under a magnifying-glass as delicate, white, 

 silky threads. The ovules of these plants are also favorable for the study of 

 fertilization owing to their small size and the transparency of their parts. Other 

 suitable plants for the purpose are the species of Pyrola, Monotropa, and Torenia 

 Asiatica. 



7. For the study of the different kinds of vegetable tissues, make, first, 

 thin longitudinal and transverse sections of the stem of the house Geranium. 

 The large pith, and most of the tissues outside of the fibro-vascular ring, are 

 composed of ordinary parenchyma cells, which may be studied without any 

 preparation of the sections further than mounting them in water. Make careful 

 studies and drawings of a few of them as they appear in both sections. 



Study the epidermal cells, together with the glandular and other hairs 

 attached to it, in the same way. 



Now treat the longitudinal section for a few moments with hot Schulze's 

 maceration fluid ; rinse it in water; put it on a glass slip; cover it with a cover- 



