Teruma, 26:16, Omdim: As They Grow- Doing Mitzvos Derech G'deilasam.

The Amudim that comprise the walls of the Heichal are referred to as Omdim- עצי שטים עמדים. The Gemara (Sukkah 45b) interprets this to be a rule for all mitzvos- that when we use an object for a mitzva it should be erect, meaning in the direction it grows. 

אמר חזקיה אמר רבי ירמיה משום רבי שמעון בן יוחי: כל המצות כולן אין אדם יוצא בהן אלא דרך גדילתן, שנאמר: עצי שטים עמדים. תניא נמי הכי: עצי שטים עמדים שעומדים דרך גדילתן. 

This does not mean it should be held as it actually grew. Esrogim, for example, grow in all directions, and only rarely do they grow with the pittum upwards. דרך גדילתן means that we hold it or place it with the direction of its growth upwards.

(The Magen Avraham in 651 brings from the Maharitz Geios and the Elya Rabba in the name of the Sefer HaManhig that it's called derech gdeilasan even if the esrog grew pittum down, and they give the reason because the Esrog begins growth with the pittum upwards and only afterwards bends down because of its weight. I've grown hundreds of esrogim, and that's just not true, so I prefer my pshat. As you'll see, it shtims with Reb Moshe's derech also.)

Reb Moshe, brought in the Kol Rom II, gives a beautiful interpretation of this mitzvah. 

יש ליתן קצת טעם לזה הלימוד, משום שהמצוה צריכה להועיל שהאדם יתגדל על ידה והיינו שיתגדל מצד מצוה גוררת מצוה אבל לא שיעשה המצוה ליפטר מחיובו, שזה אינו לרצון.

We do mitzvos not to free ourselves from the obligation mitzvos create, but instead so that we will thereby create more and more obligations, more opportunities to do mitzvos- מצוה גוררת מצוה. Mitzvos are not static, they are dynamic. One does them in the hope that they will stimulate growth.

My son Harav Moshe showed me the Rabbeinu Chananel in Sukkah 45b, who says as we are saying.

כל המצות אין יוצא בהן אלא דרך גדילתן העיקר למטה (והצמר) [והצמח] למעלה שנאמר עצי שטים עומדים. כלומר כמו שעומדין בגדילתן. 

(By the way, you should know that the elided word in Rabbeinu Chananel, (והצמר) [והצמח] , is not necessarily incorrect. In Yechezkel, the word צמר is used to describe the crown of a tree.