Abstract: The Horse-tail pine, Pinus Massoniana Lamb., commonly distributes in the south of the Yangtze-kiang. It is also cultivated on our campus. In the present paper, the writers have studied its wood formed during the youthful period(Phillips)and t(?)ied to discuss the influence of habitat factors on it. The site(Fig. 1)that the pine grows on is a hill of even slope, and covered with dry, sandy red loam originated from granite. Summer comes here from April 20 till October 31(Chen and Shen) each month with average temperature>22℃. I'rom April to September, it is the rainy season with rainfall reaching 81.1% of a year during this period, the prevailing wind is southeaster, following by souther. And the relative humidity has the maximum monthly average>80% in March-August. This pine forest is a pure one, single-storeyed and 24 years of age. Owing to thecrown density is between 0.4 and 0.6, lhe ground vegetation composed mostly ofhtqiophilous herbs, i. e., Isehaemum aristatum Linn, Eriaehne palleseens R. Br., etc. llowever, under the canopy, shrubs such as Rhodomyrtus tomentosa Hassk., Mussaenda pubescens Air., Eurya chinensis R. Br., and Baeckea fruteseens Linn. may occur. Wood samples were distributed into three ring groups, i. e., the 9th and the 10th, the 14th and the 15th, the 19th and the 20th growth rings. Above all, they were taken at two different heights.(Table 2). After the wood has been examined and described in detail, some anatomical features may be concluded as follows: The latewood portion is not easily determined when tree was younger, e. g., the 15th growth ring In the 20th ring, the proportion of latewood is higher(Table 3). Lines always present in earlywood(P1.I Fig.3,4),especially for the early ling (Table 3). The tracheids of the 20th growth ring generally have thicker wall and wider lumen, particulary in the latewood portion(Table 4). Proportion of length of tracheid in contact with rays obtained by the count method from transverse sections is larger iniatewood(Table 4). It is similar to ray number(Table 5, cf. Table 6). Both varialions in tracheid length from within outwards and comparison between latewood and earlywood tracheids correspond with observations on related genera or similar species made by other authors.(Table 4a, Fig. 2.) Pray tracheids present with various arrangements, it is worthy to mention only when they occur in continuous rows equal-in number along both margins of a ray (Table 5). They are more developed in the 15th ring, but on the contrary the following feature exists. In earlywood of the 20th ring, it has a tendency to possess a regular, large, window-like cross-field pit with narrow border, especially for the wood has no lines or false rings. With respect to the distribution of normal vertical resin canals the latewood portion has only 0.1 ducts more than earlywood; of course, younger tree may contain more ducts. The proportion of uniseriate rays to the normal horizontal resin canals is pointed out, also of the latter to the vertical ducts. (Table 6.) Referring to summer rain have an important influence on annual diameter growthof Pinus radiata I). Don(Fielding and Millett)and to variations in summer wood responded to summer rainfall(Paul and Smith; Schulman), we know that ring width of the present material is rather wide, consequently latewood percentagemay be affected, and a phenomenon is that earlywood is often met it with lines or sometimes with false ring. Based upon the characters mentioned above, we concluded that both wall thickness and lumen width and the length of tracheids, and the numberof normal vertical resin canals vary with tree age and in different portions of the wood.